If amplitude of a message signal is kept fixed and frequency is increased then modulating index will decrease? According to bessel function side tones will be more spaced if frequency increases but if modulating index decreases then amplitude of central tone frequency should increase (if modulating index goes below 2) then how bessel 0 point is achieved?
Narrow Band FM, spectrally, looks similar to AM, in that only a carrier and two sidebands carry most of the energy.
Have you defined the "data eye" your system requires, to have acceptable Bit Errors?
Once you have done so, the presence or absence of the further_out Bessel components will become easier to compute.
I think the modulating index is also called "beta", and is the phase deviation.