I have 12 rigs in my house, each with 3-4 cards (with the exception of 2 of them only having 2).  Each of my 20 amp circuits have 2 rigs hooked up.  Some of these rigs are in an upstairs room where I had an electrician put in a small sub-panel.  There are two rigs that are just in spare bedrooms hooked up to a normal 15 amp circuit each having just two cards.  

The subpanel takes up two slots on the main panel, but it is using a much higher fuse - 50 amps I think.  I am completely out of spaces.  If I had more power and ways to cool them I would put in more rigs.

I have been mining since April of 2013 and am in for $42,000 including hardware and electricity (and a couple of Monarch preorders).