Either one or a group of at least 3 (more spreads out the aggression better, 4-5 is probably good for that tank). If you get just 2 (and they aren't a pair), they will go after each other and the less dominant one will be harrassed to death. I'm having this problem now because my dominant angel died recently, leaving me with only two. A single angel is also fine alone if you don't want to fill the tank with them... plus there's no chance of breeding and the aggression resulting from it.