The number of core electrons present in Calcium is

For any atom of element, inner shell electrons (except valence shell) are called as core electrons. Generally these core electrons act as screen for valence electrons to protect them from the nucleus attraction force.

${}_{20}\mathrm{Ca}\to \underset{\mathrm{Core} \mathrm{electrons}}{1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^6}/ \underset{\mathrm{Valence} \mathrm{electrons}}{4{\mathrm{s}}^2}$

Hence, for Ca number of core electrons = 18.