The total numbers of electrons in Cr atom for which m = 0 :

The electrical arrangement of chromium is $1{\mathrm{S}}^2 2{\mathrm{S}}^2 2{\mathrm{P}}^6 3{\mathrm{S}}^2 3{\mathrm{P}}^6 4{\mathrm{S}}^1 3{\mathrm{d}}^5$. So subshell's m value is 0, p subshell's m value is 1,0, 1, and d subshell's m value is 2, 1, 0, 1, 2. Consequently, p has 1 orbital with m=0 and 1 d orbital with m=0. Therefore, the total number of electrons will be 2+2+2+2+2+1+1=12.