In a water cooled config the cards only take 1 slot. I’ve got 2 3090s and am buying another two shortly. Preemtively upgraded the power to 220v, found a 2kw PSU, and installed a dedicated mini split. I’m also undervolting the cards to keep power and heat down, because even 2000w is not enough to run 4 and a server grade CPU without tripping. When you start accumulating GPUs you also run into all kinds of thermal and power problems for the room, too.