Yes, you can keep just one angelfish. If you only have a small tank available or if the angelfish is very territorial or aggressive, then it’s a good idea to keep them alone. Just make sure to provide its tank with lots of plants and decorations to interact with to prevent it from getting bored.
Angelfish are still social animals, so most angelfish thrive better when they have friends. You should try to keep five to six angelfish in a 55-gallon tank or bigger. If your tank is smaller than that, you should keep a pair of angelfish.