The alternator generates voltage and current at any RPM, but has to reach a minimum level to charge the battery. The way it works is this:
In the absence of a load (no current being drawn), the voltage increases directly with RPM. However, the car needs a constant voltage, so the alternator output is connected to a voltage regulator which maintains a constant output voltage. If the alternator voltage is less than the regulator output voltage setting the regulator will either output no voltage or output the reduced voltage of the alternator (depending on the design of the regulator). In any case, if the voltage output of the alternator is less than the battery voltage, the battery will not be charged. This establishes the absolute minimum RPM needed--it is what ever RPM is needed to output the battery voltage (actually somewhat more is needed because of voltage loss in the regulator, and the fact that for current to flow into the battery, the voltage applied must by higher than the battery voltage.)
