You don't need separate anemone for each clown fish. One large on for the harem is fine. More important is the fact that you ask this question, meaning you don't know much about anemone.
Clownfish and anemone relationship are species specific. Anemone just don't host every clown fish. Do research on natural hosts. In aquarium, sometime anemone host un-natural species of clown fish, but not always or all of them.
Anemones are trickly to keep. Please research on them, which species and if you can provide the condition for them first before you buy the anemone.
Lastly, least important, the anemone is the host, and the clown fish is the resident, not the other way around.