Computer room/CRAC/HVAC cooling is typically measured in "tons" (for instance, my company's data centers has 15, 20 and 30 ton cooling units depending on room design). The general rule of thumb formula is:

12,000 BTUs per hour per 1 ton of cooling

The number of watts is written on the back of the power supply (and in the server's technical specs). The formula to get from watts to BTUs is:

BTU/hr = W * 3.415

So, say you have a 500 Watt PSU operating at an estimated 80% capacity. You would need:

500 * .8 * 3.415 / 12,000 = .11 tons of cooling.

OK, so now say you have a data center full of 400 of the same servers using the same PSUs:

400 * 500 * .8 * 3.415 / 12000 = 45 tons of cooling.

This is all back of napkin, but it's surprisingly difficult to get hard and fast rules for this stuff. This is all "tons of cooling", but how you have you machines arranged affects this a lot too. Hot/Cold row design helps a ton. If you get really overloaded, the special cabinets that have duct work in the front door and a heat vacuum in the back can really cram more heat into an area.