The guide for adult angels is 10 gallons apiece. In my experience it is best to follow that as a minimum, although you could probably add a few otos to your tank for clean up as they aren hard on the bioload.
"Careful. We don't want to learn from this."
- Calvin, Calvin and Hobbes
id say only 2. becuase if 2 of them breed one will get picked on and will be like outcasted or whatever. if none of them breed you'll probally be fine with 3. if you add 4 you'll have fights between the 4 fish about protecting their eggs and whatnot.