The biggest known unknown is how many riders to expect, and restricting numbers seems a sensible idea to begin with - what was your limit?
Also bear in mind that you will have at least 10% DNS.
In other words, if you think you can handle a field of 50 riders, set your entry limit to 55. (And don't run a 'waiting list')
Cynical - moi?
And 10% will DNF so make it 60...
But consider where the bottle necks are. Unless you specifically want to limit entries, 50 as a limit is actually quite small, especially for events during the time of the year when you can reasonably expect it to be vaguely warm and dry, i.e., riders will not be huddling inside controls to escape the weather.
The immediate bottleneck is at the start, handing out Brevet cards and/or vittals.
Assuming an indoors start, lay out labelled cards on a table and riders will help themselves.
If it's a village hall, recruit 1 or two helpers to dish out hot drinks from the early arrivals... you'll not be short of volunteers. Paper plates and cups eliminate washing up. Lay out whatever breakfast fare you fancy (biscuits, toast, jam, whatev.) and those who want will help themselves. On the road and at the arrivee, riders arrive over a period and arrangements for looking after 50 will generally equally serve 100.
Whats the point of this? Well, roughly
70% of all events in 2017 had <50 riders, and
90% had less than 100, and those with more were mostly club/ctc supported 100km BPs.
So any concerns new organisers might have about being inundated with entries are simply misplaced.
Think Bigly