  We investigate the causes of the different shape of the $K$-band number
counts when compared to other bands, analyzing in detail the presence of a
change in the slope around $K\sim17.5$. We present a near-infrared imaging
survey, conducted at the 3.5m telescope of the Calar Alto Spanish-German
Astronomical Center (CAHA), covering two separated fields centered on the HFDN
and the Groth field, with a total combined area of $\sim0.27$deg$^{2}$ to a
depth of $K\sim19$ ($3\sigma$,Vega). We derive luminosity functions from the
observed $K$-band in the redshift range [0.25-1.25], that are combined with
data from the references in multiple bands and redshifts, to build up the
$K$-band number count distribution. We find that the overall shape of the
number counts can be grouped into three regimes: the classic Euclidean slope
regime ($d\log N/dm\sim0.6$) at bright magnitudes; a transition regime at
intermediate magnitudes, dominated by $M^{\ast}$ galaxies at the redshift that
maximizes the product $\phi^{\ast}\frac{dV_{c}}{d\Omega}$; and an $\alpha$
dominated regime at faint magnitudes, where the slope asymptotically approaches
-0.4($\alpha$+1) controlled by post-$M^{\ast}$ galaxies. The slope of the
$K$-band number counts presents an averaged decrement of $\sim50%$ in the range
$15.5<K<18.5$ ($d\log N/dm\sim0.6-0.30$). The rate of change in the slope is
highly sensitive to cosmic variance effects. The decreasing trend is the
consequence of a prominent decrease of the characteristic density
$\phi^{\ast}_{K,obs}$ ($\sim60%$ from $z=0.5$ to $z=1.5$) and an almost flat
evolution of $M^{\ast}_{K,obs}$ (1$\sigma$ compatible with
$M^{\ast}_{K,obs}=-22.89\pm0.25$ in the same redshift range).
