I asked this question of other showgoers at AXPONA, and in my informal and non-scientific anecdote collection, the answer seemed to be between 2 and 3 systems on average. Admittedly, this is a sample of a set of enthusiasts.
I have three systems posted, along with some gear not in the active systems. That does not include my AV system, because I really don't invest any time or thought into it. The household name for that space is The Brain Melting Room, once the TV is turned on.
I like extra systems to be able to experiment with pieces, while looking for the best combinations. Folks talk about synergy, but it's very interesting when things fall into place. As this happens, the turnover of gear seems to slow down, with each system offering a flavor in its particular space.

