You should stay away from putting too many PWM fans on one header as they constantly pull 12v, I would instead get one of the splitters that expands the 4th pin to many fans and gets 12v directly from the computer's power supply. In PWM mode the fan is fed 12v constantly, and then a PWM signal is sent up the fourth pin to the fan to control it. PWM control is also used in your VRM and throughout your motherboard in many different places. If you feed a fan 12v constant and use a 50% duty cycle PWM rate, then you would theoretically get 6V. That would result in a 50% duty cycle since you are pulsing (turning on) half the time. So let's say in a 10-second cycle we pulse five times to 100% for one second each time. These pulses result in a duty cycle, which can be used to calculate final voltage.

PWM Mode (4-pin fans): Stands for pulse width modulation, and it basically means you get pulses (basically going to an ON position) at a periodic rate, and in this case, it's thousands of times a second.

Asus rog fan speed