I've done a lot of reading on Betta breeding; I want to try my hand too :P And from what I gather, the more "virgin" pairings usually have relatively small spawns, anywhere from 5-50. But that they can also have normal size spawns. Normal spawns can be anywhere from 50-1300 eggs, though there's only a 30% survival rate...so out of a spawn of 500 you might have 150 that live to adulthood.
Hope that helps. :)