I think I could actually be really happy with just one until they became fairly senior. I'm not good at splitting my loyalties, and I always feel terrible when I take Gusto for a day of training or trialing and leave Meg at home - and I never, ever leave overnight with him and leave her home.
Looking ahead into the future, I worry that when I have a puppy, I will feel bad taking it out for all the regular training I would want to do, and leaving Gusto or Meg behind. Especially if it is Gusto left. Meg is really fairly content to stay home, but Gusto is quite sure he is supposed to be invited on all adventures.
The thought of having just one again, to completely bond with and take everywhere, is actually really appealing. But then I think two, because my two really do adore each other, and I think having a nice stable adult dog while raising a puppy is good for everyone. But then I think maybe three - one retired senior, one active competitor, and one up and coming puppy.
Three would probably always be my max. Two is probably my realistic answer. One would be perfect, if they promised never ever to get old.