dude  there are many rigs.

a few   can use 3 psu's   the s-9  can do that.


now gpu rigs with  risers  can use many psu's  if wired correctly.



some gpu rigs  have 19 gpus's   in theory you could use  more then  4 psu's if you wire it correctly

In all the cases above if you wire wrong pop you fry the gear