Well, both really. The cell count determines the voltage going into the motor/ESC. Voltage determines the RPM that the motor will turn, and the load on the motor determines how many amps it will pull. If you put too much voltage for the load (prop) you are using, you'll burn up your motor or ESC with too much current. So motor combos are rated for a certain voltage range which will correspond to a certain Lipo cell count since that's what nearly everyone uses now. Often, a motor's specs will give you the right props to use with each battery cell count to keep the amps down to what the motor can handle.