No, just one. But he can change back and forth as necessary.
I don't believe he's actually controlling anyone, just manipulating his body to look like them. For instance, he appeared as Walt when Walt was no longer on the island.
>>>>>There's no proof that he was Walt or Alex. Only Christian,Yemi and Locke. What those three have in common is their dead bodies ended up on the island but they didn't die on the island. I don't think he can use bodies that died on the island, cause if that's the case there are plenty of dead bodies on the island to choose from. I know!!! but dreams don't count.
>>>>>>>>>Cerberus can manipulate himself into anyone or anything that affects you in order to get his answers, in order to see if you really are repenting or making progress towards your "leaving" or "moving on". Yemi was Eko's test, which he passed? Even though it shouldn't have been that violent. Eko's journey ti self discovery led him there. Walt was a manifestation for Shannon, an effort to keep her "good and unselfish" and possibly led her to her death by Ana Lucia because she had also proved her worth.
^^^ incorrect about walt. It is asked of Michael if Walt was "ever somewhere he shouldn't have been" Walt was really there as Saynid saw him as well. Ben asks "what's stoping it from changing the way he looks" "Bc he is stuck that way" It is never explained why he is stuck that way, why he can look like christian or locke. It is possible smokey was other people but the show has only confirmed that he has looked like himself, locke and christian.