In a nutshell, the problem is that there are several different types of 'Waypoint' and any documentation and discussion around this subject is usually insuffiently clear about which type is being referred to at any given moment.
The Waypoint memory limits of the GPS are documented in the specifications, but some wrinkles only become clear on reading through the manual, or by trial and error. The limits can't be overcome by fitting a bigger memory card - the waypoints, routes and tracks are all held in on-board 'data memory' - only the maps are stored on the microSD card, asuming the GPS is equipped with one at all (that is, all the 'x' models).
A typical Garmin mid-range GPS Waypoint Spec is:
Etrex Legend Cx
500 Waypoints
50 Routes
Each Route can have up to 250 waypoints (off-road or direct routing mode)
or up to 50 waypoints (follow road or autorouting mode)
10000 Trackpoints
20 saved Tracks
Each Track can be up to 500 trackpoints.
NB that these limits have nothing to do with, and are not affected by, the size of any SD memory card that may be fitted. Nor do any extra POIs (Points Of Interest) uploaded into the GPS, impinge on any of this.
Some units have lower specs:
The Etrex C series can only store 20 routes.
The Geko 201 is limited to 125 waypoints per route and 20 routes, and can only store 10 tracks.
Some have higher specs:
The 60 and 76 and Colorado/Oregon series, and the Etrex HC series, can store 1000 Waypoints - with these units this 'problem' (which is only a misconception anyway) kinda goes away.
Is 500 Waypoints enough? - it looks as though the Legend C can store 2 routes of 250 waypoints each, or 20 routes of 25 waypoints each, or of course any combination between those extremes, for example 5 routes averaging 100 waypoints each. Typically in laney terrain and using direct routing mode, 100km will require about 100 waypoints, give or take. So there's no problem for day rides, even if you have to split the route in 2 or 3 to avoid hitting the 250-point limit - but on the face of it, it looks as though you could struggle to direct-route your way around a laney 600, or indeed a week-long tour.
But the answer is, YES 500 is ample - because there's some sloppy terminology going on here and its a case of apples and oranges.
If I re-write the first few lines of that typical mid-range Spec above, changing the terms slightly - does this make it clearer? -
Etrex Legend Cx
500 User Waypoints
50 Routes
Each Route can have up to 250 Routepoints (off-road or direct routing mode)
or up to 50 Routepoints (follow road or autorouting mode)
A User Waypoint may or may not also be a Routepoint, and vice versa,
a Routepoint may or may not also be a User Waypoint
In an extreme example, a GPS so specified could contain 50 Routes of 250 Routepoints each, totalling 12,500 points which equates to about 12,500km of navigated road - without having a single User Waypoint stored in memory. Or it could even have up to 500 User Waypoints stored as well, which may or may not figure in the Routes.
Hard to prove this extreme example - but I can certainly vouch that the Legend C (an older model with less memory) can store and follow 12 routes with an average of over 100 points per route - totalling around 1300 waypoints or more than 2.5x the specified 500. This was enough for a 17-day tour through France with plenty of capacity to spare.
It's as simple as that really - 'waypoints', which are really just a stored pair of co-ordinates - come with added baggage that makes them into Trackpoints, or Routepoints (sometimes called Via Points), or Map Points, or User Waypoints, or POIs (Points of Interest), or Geocaches. (See footnote below for an attempt to describe these different points in more detail.) The Garmin can recognise and make use of all these different types.
A User Waypoint can be very information-rich, compared with say a Trackpoint or a Routepoint, so there is a limit on how many can be stored. But all that is needed for Routes is Routepoints (up to 250 per Route) and all that is needed for Tracks is Trackpoints (up to 500 per Track). User Waypoints are not needed at all, but are often used as Routepoints because, being information-rich, they can supply extra information. In the Garmin, they also work much better than Routepoints if the Route is intended for autorouting (Follow Road) - at the very least, the destination (final Routepoint) should be a User Waypoint.
If you construct a on-road Route by 'joining the dots' using the Route Tool, in either Mapsource or Memory Map (and maybe other programs, I don't know) the points laid down are Routepoints. Often, they coincide with a road junction in which case Garmin calls these 'Map Points'. They are not User Waypoints unless one happens to coincide with a User Waypoint that has already been laid down.
Map Points have rather limited functionality compared with proper full-blown Waypoints, and most noticeably in Mapsource they cannot be renamed (though they can in Memory Map). I like to use named waypoints at significant junctions or at any point where it looks as though navigation might be tricky - a slip-road and underpass for example. So my approach to routing is to lay these User Waypoints down first (using the Waypoint Tool) and name them, and then to use the Route Tool to join the dots including any pre-laid waypoints. The end result is a Route that is typically around 50% User Waypoints and 50% Map Points. I never go above 125 points in total, because I like my routes to be downwards-compatible with the Geko, which has this limitation.
If planning an itinerary that uses 8 routes or more (such as a 2-week tour) then I do still start to bump up against the 500 User Waypoints limitation. In such a case, I'll simply be more sparing with my User Waypoints in the first place (they are mostly a luxury) - or, recently, I've adopted a new workflow:
It is actually possible, after using a 50-50 workflow like the above, to strip out some or all of the User Waypoints leaving only Route Points - but named the way you want them. The resulting Route is completely functional in the Garmin, and takes up zero User Waypoints (in an extreme example).
Just save the GDB file as a GPX. In a text editor, strip out all the text after the tag [/metadata] and before the first instance of the tag [rte]. Save, and open it in Mapsource, resave as a GDB. You'll find the Route is still there, but all the Waypoints have gone!
I use this tecnique to construct routes with about 50% named waypoints, but only about 10% actually left in as User Waypoints. This works out at about 10 User Waypoints per 100-point (average) Route, so I could actually store up to 50 of these, before hitting both the 50-Route limit and the 500 Waypoint limit. But I've never yet found a need to store more than about 17 Routes.
NB - this is all assuming direct routing mode ('off-road' in Garmin-speak). If you are planning to use autorouting ('follow road') then you would obviously construct the route with fewer points anyway, within the 50-point limit. (And you'd be well advised to make a good proportion of those, User Waypoints.) Attempting to use 'follow road' with a route designed for 'off road' (or indeed vice versa) is just not going to work very well.
If you prefer to navigate by following a Track - then notice that there is a serious limitation of 500 points for a recorded Track, even on the most advanced units. That only gets you about 30km. You have to downsample longer tracks, which works OK up to a point. If you upload a longer track into the GPS it will be truncated. Again, you could split it into several, of course.
Footnote: Waypoint types: (NB - just my take on this confusing subject)
Waypoint is a generic term for a pair of co-ordinates in a wrapper of code. Usually there will be at least one other data field within the wrapper, which would be a unique Id or Name. There may or may not also be several other data fields, and see also 'User Waypoint' below.
The Waypoint is one of three top-level elements of GPS navigation, the others being the Route and the Track.
Route is a term for a collection of Routepoints, listed in the order in which they are to be visited. (But NB that 'Follow Road' mode in the GPS can override this order and even disregard Routepoints altogether.)
Track is a term for a collection of Trackpoints, listed in the order in which they were written.
Trackpoint is a Waypoint but it doesn't even need a Name field. To be a viable point, it has to be part of a Track. Trackpoints are generated by any software that has a 'Track' tool or 'Draw Track' option, and they are also generated by a moving GPS as a 'breadcrumb trail'.
Routepoint is a Waypoint that is part of a Route. (One implication of this is that the Name is unique within that Route, but if there are 2 or more Routes, the Name need not be unique overall.) Routepoints are commonly just called Waypoints. Routepoints are generated by software that has a 'Route' tool or 'Create Route' option. The tool may or may not simultaneously generate User Waypoints (see below).
Via Point is (as far as I can tell) just another name for a Routepoint.
Map Point is a point (not strictly a waypoint) that constitutes an element of a vector map. Like any other point, it can be information-rich and Garmin's system makes extensive use of this information to generate navigational hints. Garmin's Route Tool uses Map Points wherever possible, copying them in preference to generating new User Waypoints.
User Waypoint is my term to differentiate a (usually) information-rich Waypoint from all the other types listed above. User Waypoints are commonly just called Waypoints. They are generated by any software that has a 'Waypoint' tool, or on a live GPS by using the 'Mark Point' facility. Some software also generates them as a by-product of the 'Route Tool'. Garmin's Route Tool uses existing User Waypoints wherever possible, in preference to Map Points.
POIs or Points of Interest are very similar to User Waypoints but are stored as named collections (such as 'Transport', 'Hotels', etc). POIs are embedded in some maps, each collection acting rather like a map overlay. POI collections can be downloaded from the Internet or generated using a free software tool supplied by Garmin. POIs are used extensively by the GPS 'Find' facility.
Geocaches are very similar to User Waypoints but with specific data fields that mark them out as Geocaches. They are used to mark the locations of 'buried treasure' to give people a raison d'etre for travelling with a GPS. Geocaches can be downloaded from the Internet, and the GPS has a special page to handle them.
Francis Cooke
Living with a Garmin: The Waypoints Limitation
Living with a Garmin: Track, Route or Autoroute
Living with a Garmin: Struggling with GPX
Living with a Garmin: Battery Runtime and Etrex Jitter
Living with a Garmin: Living with Metroguide Maps
Living with a Garmin: Declutter the Page Sequence
Living with a Garmin: Screens you don't see every day
Living with a Garmin: Less is More
Francis Cooke's Garmin Menu Map (pdf)