Controlling multiple fans using arduino uno
One PWM pin can control only one fan (or one group of fans). You can't have one PWM pin outputting different values to different fans, since one pin can only have one value. (Yes, there are ways of multiplexing it through time slicing but that is both hard and not a good idea).

You need one PWM per fan if you want individual control of each fan's speed. That means a maximum of 6 fans on an Arduino.

To get more you would either have to use software PWM (which can introduce jitter which may not be a problem with simple fans), or an external PWM controller.  Or add more Arduinos.

If all you want is potentiometer control of the fans you can do it without using an Arduino. A pair of op-amps (it can be a single dual op-amp chip), one acting as a triangle wave oscillator, and the other as a comparator, will generate PWM for you.  Here's an example:


  
  
  Image by ElectroBOOM (watch his videos sometime, they're hilarious)