I have 10. I have one each on Materia, Light and Chocobo for access to the other regions to tour housing. I have one on another NA world to play with relatives. Then I have the rest on Coeurl. Most of them were created just to replay MSQ but I got a bit attached to them along the way and never deleted them like I normally do when I'm done.

If you're an alt lover, I don't recommend having more than 3 that you play seriously unless you're able to truly no life the game. It gets to be difficult to keep up all of your alts on everything that interests you. Three is also a good number if you're into gathering to get maximum use out of the 2 hours timed nodes without severely straining GP available for the next node.