You can keep several in a 75G until they start pairing off. I probably wouldn't go more than 2 breeding pair in a 75G, and even then, there can be a lot of aggression.
Several as in around 12 max. would fit (they get to 6 in. I believe, fully grown). But then like lonewolfblue said you probably shouldn't really have more than 2 pairs or they may get aggressive. Fill it with some sort of schooling fish along with the angels.
