I am of the personal peference that there should be 1 psu per rig, as this makes it a lot easier to manage and diagnose .
There are some chinese brand 3600 double atx psus available that can run many cards
 But yeah its more of a peference , Cause in my country its really cheap to get 5-6 slotted 1-2 gen i5 cpu + processor + 4 gb ram + 160gb hdd  in 50$
and these are old sli boards and u can run  2 cards directly on board (although not recommended if you dont have proper support for cards )