Graynomad:
I can use like 8 tiny's or something
Yes you can, I helped a mate do just this albeit with only 4 slave processors.
OTOH if there are chips designed for this they may be a better option.
Now the next thing to think about is how often do you need the data to be sampled? Can you just sample one motor at a time then move onto the next one and sample that? If so then all you need is a MUX.
Rob
That was my other thought. For 66 rotations per second, you can poll the motor for 1/10 of a second, and capture 6.6 rotations. 40-1/10ths of a second is 4 seconds to poll all of them.
If a 4-pole magnet is used, that's 26.4 clicks in 1,600,000 cycles of the CPU. The clock would only capture the 26, so there'd be an error of (26 * 10 * 60) / 4 = 3900 RPM = 100 RPM error.