We have 12 tanks, all over the house. Most are at least passable in terms of how they look, even the "breeding" tanks.
We put all our fish equipment in the wet bar that we don't use, but a set of shelves or a closet would do just as well.
Shelving units need to be really well thought out in terms of making maintenance easy. A long unit (like a really long stand) with all the tanks at the same height might work for just 5 or 6 tanks.