1 for PSU usually
1 for CPU almost always
1 for GPU almost always
1 for case near the back/high/mid area for exhaust.
There should also be a 'vent' over the video card so that it has access to relatively cooler outside air (even if the room is >= 35C, that's cooler air than the GPU's heatsink).
There's an ATX specification by Intel that covers this sort of stuff btw.
There are also a lot of 'ATX cases' that aren't really compliant and kill peoples computers slowly.