How to compute number of samples in 1 cycle of signal?
Although it is hardly related to DSP but here is a quick calculation: 

$R$ Rotations per minute is $\frac{R}{60}$ Rotations per second which gives $12K$  samples per second. Hence, One rotation is proportional to $\frac{1}{\frac{R}{60}}\times 12000 = \frac{720,000}{R}$ samples. For instance, for  $2000$ RPM there are $360$ samples per rotation.