Usually a few each generation. But it depends on the orientation.
The old style had the PSU intaking from the area around the motherboard with a downdraft cooler on the CPU and exhausting out the rear top. A good configuration when chassis used to come with one or zero fans. Something to get the heat out the chassis. I even had my power hungry GTX580 in an old Cooler Master case with that arrangement. Never caused a problem.
I have seen top mounted PSUs that are in a separate chamber, so don't act as part of the airflow system. Basically a tiny PSU case above the motherboard.
There was one a year or too back that had the PSU facing up, drawing air from the outside and out the back. Top intakes not really a great idea in my opinion. But that was a strange case with the PSU featured kind of prominently. I will see if I can find.
Behind the motherboard is another popular one, and that puts them still relatively close to being top mounted from a physical perspective.