There is no fixed formula for calculating the gain needed to modulate an RF signal. You have to amplify the small electrical voltage or current from the microphone to a strength that's sufficient to modulate the RF signal. So, the amount of gain necessary depends on the output from the mic and the RF power at the point of modulation. The output of a microphone depends on the sensitivity of the mic and the loudness of the sound it picks up, and may be anywhere from less than 1mV to 100mV. In broad terms, a mic signal may be considered to be of the order of a few millivolts.
Thus, the amount of gain needed depends on three factors:
1) The strength of the mic output.
2) The strength of the RF carrier at the point of modulation
3) Modulation index.