The graph below gives an indication of the size of entry you might expect for your event.
Over many years it has been found to hold remarkably true for events large and small - 20 starters or 200.
It is based on a graph originally published in Arrivée by Keith Matthews, organiser of the Dorset Coast 200.
(Arrivée, September 1984, page 19).
Basically it suggests that, when you reach the 2-week deadline for ordering brevet cards, you should seriously consider the possibility that your entry could double by event day.

Days to go
-50
-48
-46
-44		6 weeks to go
-42
-40
-38
-36
-34
-32
-30		1 month to go
-28
-26
-24
-22		3 weeks to go
-20
-18
-16
-14		2 weeks to go!
-12
-10
-08
-06		1 week to go!
-04
-02
-00		Event day