The number of angels you can get away with all depends on the personality of the fish. I have a 55 and it is big enough for ONE. That's because, no matter how many other angels are in there, the tank is HIS. Been that way since he was the size of a nickel. That's why he killed 5 other angels (all introduced in groups). Originally, he was put in with 3 other angels. He killed 2 of them over the course of a few weeks (tank had previously been cycled). Then, not wanting to leave 2 in there, I bought 2 more. The constant harrassment and stress of adjusting to a new tank killed those 2. Later on, I introduced an angel of comparable size to spread out some aggression. He killed that one in 2 weeks. The one remaining angel that wasn't killed recently had to be pulled out of the tank after a year and a half of being bullied. He almost died from the stress.
So, be careful. And, if you're going to introduce more angels, do it ASAP. The longer you leave your one in there, the more it will see the entire tank as "theirs."