Snowbirds is a different beast compared to your Club Racing. Snowbirds pretty much HAS to fill the heats. with 130 racers in stock last year..... you get the picture. Normally if you have more then 1 heat, divide them up so there is a nice mix.
25 Entries... 3 heats
35 entries....4 heats
I wouldnt go less then say 6 or 7 per heat though