It is explained in that rcgroups article - the number of poles corresponds to the number of voltage cycles per rotation (logical, the voltage changes as many times as many magnets pass in front of the coil).
So take the double of measured frequency, divide by the number of poles and multiply by 60 to get RPM value. However, this is going to be a fairly noisy value, depending on how exactly are you measuring the BEMF.
BTW, you need to count poles of rotor (magnets!) not the stator poles. 12N14P is a 14 pole motor (the number before the P - again, did you read that rcgroups posting? 12N - 12 stator windings, 14 magnets), so:
2*1600/14 * 60 = 13714 rpm, if I am calculating it correctly.