If you have room for a 29-gallon, that would be better, but a 20 gal is doable. The tank should have many hiding spots. As FishandBirdLover said, it should be 70-75% planted. Tank mates are a must because they spread out aggression. I have heard that rummy nose tetras are good tankmates.
Additionally, you should have a spare tank in case there is one bully betta. Another thing is to make sure that all your bettas are female. I know it's quite obvious, but I have heard horror stories of one of the bettas being male. I believe the best number of fish for a 20-gallon is 5 bettas. I don't believe there is any evidence about this, but I have talked with people that having different color bettas helps with aggression. Lastly, if you want more information, try watching videos of KGTropicals on youtube.