As many as you can LOVE, have room for, have time for, can afford (food, vet, preventitives, toys, clothed, etc...), and have energy for!!
I hope someday I can have at least 3 or 4, now I just have to see if Missy agrees...
she may be a one yorkie and that's it kind of gal. Hopefully socialization will help her want another furbaby around!!