I am actually using an external 12 V power source to power the fans. The fans I am using is Delta 12 v dc fanThis has two pwm wires, white and green for the two fans in series.
1. You said "One fan is too much for an Arduino pin! It is not working fine", this got me confused, Even one fan is too much or you meant two fan is too much?
2. If I use multiple fans, the total current to drive the fans needed will increase, but that is coming from an external power source. I have included a sketch to show my connections, for example, one fan (made of two smaller fans in series) connected to pwm pin 4 of arduino mega. Are you talking about current in pwm pins?
3. Can you please elaborate on negative voltages part? Does it mean that the voltage at the pwm wire on the fan side is sometimes higher than the arduino side and current flows into arduino from fan through the pwm pins?
4. If I use one fan per pwm pin, fans powered externally, do I still need the diodes and transistors?
the OP clearly states that they're using computer fans with a PWM speed control wire
No, the original post was ambiguous. If you read that post in isolation, as I did, all it says is "I need to control 10 fans using pwm signals". It does not say "I need to control 10 fans that have pwm inputs".
My bad - I'm not sure how I hallucinated that...
The data sheet for the fan showed that the control circuit has 10K pull-up and 20K pull-down on the pwm input, and feeds the gate of a MOSFET, so the current should be pretty low.