Lillianfish, I see you just joined us, so welcome to TFK forum.
And, as a new member, you are not likely aware of our fish profiles, second tab from the left in the blue bar across the top of the page. Information on numbers, minimum tank size, compatibility issues, water parameters, etc are included for each species. If the common or scientific name is used in a post exactly as in the profile, it will shade and you can click on the name for the profile, example Pterophyllum scalare for the common angelfish.
You will note in the profile it mentions that angelfish are shoaling fish, living in groups, so this is the best way to maintain them. And 5 is about the minimum number and in a 4-foot tank like a 55g or larger. A mated pair that you intend to breed can be kept in a smaller tank, on their own, say a 30g, but this is only as breeding and not intended permanent. You will find other useful info in the profile if this fish is still one you would like.