High end zoanthids are usually sold by the polyp. That is one individual, like this frag appears to contain about 16 polyps:
If you are not looking for the more expensive zoanthids, you can usually buy them on rocks with 100+ polyps. In cases like that, the individual polyps are not counted, but it will usually say something like a softball sized rock, a golf ball sized rock, etc to give you a rough idea of how many polyps are included.