Personally, I say one sheep is too many.
No, really. When the new Pastel started out, there were three sheep characters in town! Three! But two eventually moved, and, recently, so did Joey. I use to hate it when animals left in Wild World, but now I'm hoping they'll move to make room for the ones I grew attached to in the last game. You can complain about the animals in town hall, why not make one of the options to request that they move?
Or maybe you could threaten them with bug nets. Animals don't seem to like those.