Follows Bessel function.
There is infinite series but generally the number of significant peaks is dependent on modulation index.
Mod Index of 0.01 -> J0(beta) = 0.995 (carrier component), J1(beta) = 0.005
Mod Index of 2.0 -> J0(beta) = 0.220 (carrier component), J1(beta) = 0.580, J2(beta) = 0.35, J3(beta) = 0.13, J4(beta) = 0.03 ; has 4 significant sidebands with RF bandwidth of 8 x freq of modulation.
One accurate way of measuring modulation index is to look for particular sideband nulls.