You can buy as many heads (polyps) as you want. Some zoa's grow fast and some grow slow. Do some research and find some that you like. I have about 5 different kinds of Zoa's right now in my 29g, I usually buy small rocks with only about 5-10 polyps on it, so that they spread onto other rocks, and i can add more kinds of them. Its really up to you on what kind you want. Good luck. =]