  We present a time-dependent variational approach with the multiple Davydov
$D_2$ trial state to simulate the dynamics of light-matter systems when the
field is in a coherent state with an arbitrary finite mean photon number. The
variational approach captures not only the system dynamics but also the field
dynamics and is applicable to a variety of quantum models of light-matter
interaction such as the Jaynes-Cummings model, Rabi model, and Dicke model, and
is feasible to tackle the multimode quantized fields. By comparison of the
variational and semiclassical dynamics of both the system and field, we
illustrate that the variational dynamics from the quantum models agrees with
those from the corresponding semiclassical models as long as the mean number of
photons is sufficiently large. Moreover, we illustrate that in the crossover
between the quantum and semiclassical limits, the quantum corrections lead to
the collapse of the oscillations in dynamics, which is absent in the
semiclassical models. The variational approach provides a unified treatment of
light-matter interaction from the quantum to the semiclassical limit.
