Been here, done it
Worked out well for me, i run 2x fans on 1 PWM header and another x2 fans on another PWM header.
Like people have said already, find out what the header will support in load, then find out what your fans use. One bit of information thats important and very difficult to find is what your fans use on first spin up.
When you first spin up a fan it will actually use a higher load to first get turning, then load drops. This information of load on spin up is...