I was happy with one, and also checked it was working every day after changing water; I did put 2 bottles in when I left my ham alone overnight, just in case one failed before I got back the next day, but maybe that is overprotective mummy behaviour??
I've also read that in the more complex cages with many compartments / tubes, it can be good to have water in more than one location, to save the poor hammy having to trek back to one bottle every time.