all of my live ones I play in have 20 spots, 10 that start each week and 10 on the bench. at bare min you need the following:

2 QB
4 WR
3 RB
2 Def
2 TE
2 flex
2 k

this would be 17 rounds, and I usually like to have at least 3 flex and 4 RB since you will have multiple players on buy weeks. We could have anywhere from 17-20 rounds and as mentioned in a response below we could have a shorter clock for decisions.