Most people that buy Angelfish on impulse find out the hard way about cichlid behavior lol.
I told a co-worker her aquarium was too small a 30 gallon and 2 of the 4 she purchased were going to pair off.
After around a week of having them the co-worker said 2 were getting aggressive and she noticed eggs. Imo a solo angelfish will be ok. If you can find a proven pair get them.
Basically what I'm saying is Angelfish are cichlids and should be treated as such. If wanting to keep a group make sure aquarium has the proper space to accommodate them.