The number of tobaccos from which we alternately smoke is, of course, only a matter of preference, and only if we confess to a large number on a pipe board, no one else will likely know. Where does it say that that keeping that number low is better than allowing it to go high? It seems disciplined to keep them low, but how does a number of tobaccos open make us any better of a pipe smoker? One could argue that repeatedly smoking a certain blend allows a fuller acquaintance, and that many open blends effects the reverse, but who is to say that one who skims cannot ably taste and enjoy his tobaccos in such manner?
I've since stopped, but for a long time if a tobacco didn't appeal, it got put in a jar and put away. When I "quit" smoking in 2012, I had 50 such jars. That I wouldn't recommend.
I typically have 15-20 tobaccos open.