Four systems if we count the AV system, all in house. I think the idea came from the TAS days when there were upstairs, main room, and basement systems discussed in reviews.
It is useful for keeping items that may turn out to have unexpected synergies, and to have a variety of rooms to enjoy with music.
Selling off bits of systems now and then allows me to raise funds for the next round of mucking about. Or a nice bottle of something, Cheers.