  New light species can contribute to the number of effective relativistic
degrees of freedom ($N_{\rm eff}$) at Cosmic Microwave Background (CMB) which
is precisely measured by Planck. In this work, we consider an MeV scale
thermally decoupled non-minimal dark sector and study the imprint of the dark
sector dynamics on the measurement of $N_{\rm eff}$ at the time of CMB
formation. We have predicted the allowed region of model parameter space in the
light of constraints arising from the measurements of both $N_{\rm eff}$ and
dark matter relic density by Planck. It turns out that the impact of the dark
sector dynamics on $N_{\rm eff}$ is significant in case of a non-hierarchical
mass spectrum of the dark sector particles.
