Transmission of N signals, each band limited to fm Hz by TDM, requires a minimum band-width of :

By the Nyquist criterion, each signal band-limited to $f_m$ must be sampled at least $2f_m$ times a second, so N multiplexed signals give $2Nf_m$ samples a second. A channel of bandwidth B can carry at most 2B pulses a second without intersymbol interference, so the minimum bandwidth is $2Nf_m / 2 = Nf_m$. The figure $2Nf_m$ is the combined sampling rate, not the bandwidth.