Eric – they have mains for every class at Nationals regardless of rider count. Both of your scenarios will have a main.
They will always drop at least one rider out of motos before the main.
5 guys in class = 4 in the main.
6 guys = 5 in the main
Except for the elite classes, if there are 7 in class, you will have a gate of 4 and a gate of 3, and 5 make the main (drop 1 from each).
Someone correct me if I am wrong.