Did someone say "fan feedback?!"
I like 16 or 18, but with the understanding that the additional 2 or 4 only be alternates in the case of a blow-out or injury. That way, a skater who is an alternate can realistically tell their friends/family "I might get to skate". If friends/family decide to come to a bout to see a specific skater who doesn't end up getting used, they'll still see some awesome derby. Also, not including the full 20 incites competition and motivation for a spot on the bout roster, and including more than the WFTDA-14 gives some of the developing skaters a shot at some minutes.