I believe that nearly all computer motherboards I know of have a separate PWM pin for controlling the fan speed. That 555 controller from your image looks like it would just PWM the power to the fan, which is doable, but not ideal.
First, you need to figure out what kind of fans you have:
Fans with 3 pin connectors will typically have GND, +12V power, and a fan speed sensor. Rather than controlling the fan's speed, this just reads the number of pulses from the rotation inside the fan's motor (usually using a Hall effect sensor, I believe), which can then be read by a software program or through the BIOS.
Ideally, you will want fans with 4 pin connectors: GND, +12V, speed sensor, and PWM control.
From there you can just graft the PWM inputs of the other 8 fans onto the PWM control pin of the CPU fan. As Spec mentioned, the input impedance of each of the fans is probably quite high (such as that of a CMOS input gate), so connecting several in parallel probably won't be an issue. I don't recommend connecting all the fans to the same power pins, though, as that would be quite a lot of current for a single Molex-type pin to handle, especially in such a mission-critical location as your CPU fan.
If all your fans are 3-pin or even 2-pin (just power and GND), then you are going to need to get a bit more technical and connect a buffer in series with the PWM output from your MOBO and use the PWM signal to switch the power rails of your fans. A high side driver ship, or a level shifter to a power MOSFET would probably do the trick. You may need to add in a back-EMF diode to protect against the voltage spikes from switching the fan motors on and off-- I'm not sure if those come standard in computer fans or not.
Lemme know if you follow all that or have questions. I'm not always terribly good at toning down my techno-jargon.
Also, I'm assuming you already know this, but unless those fans are REALLY quiet, 10 fans going full blast once your CPU heats up is going to be rather loud...