It all depends on a few things like how many pumps you will be using, how you want to go about plumbing things etc. For eg, some people, rather than using say a single 2 inch bulkhead for a drain for a pump will use 2-3 smaller sized bulkheads to cut down on the suction force in the tank so you end up with 3 holes rather than 1. Also, on the outputs, some people would rather use "Y" adaptors to cut down on the number of holes they'll have to put in the tank. Just a thought
Charlie - I assumed you asked this b/c due to strength you can have more holes in one vs. the other, no? If yes, I assume thickness of the glass or acrylic would also matter.
its 12mm thick glass .i was only going to put 2 holes about 32-40mm out from inside a weir and 2 the same or a bit smaller going back to tank.
thanks tel