A lot depends on the species. Anemones like BTA's which asexually reproduce and stay relatively small can be kept in large colonies. Anemones like H. magnifica which can get VERY large and have specific lighting and flow requirements should probably only be kept singly.
The interesting thing about anemones, they either THRIVE or DIE. There seems to be very little middle ground. If conditions are right and they thrive, they can get very large very quickly. Growth can be managed somewhat by limiting feeding, but some of this may not be in your control because anemones will eat anything in the water column. If you feed your tank AT ALL an anemone will get some of whatever you put in.