The minimum number of electrons having magnetic quantum number zero for Ni is 13.
The electronic configuration of Ni is 1s22s22p63s23p63d84s2.
Except for 3d subshell, remaining each subshell has two electrons; for them, the magnetic quantum number is zero, and 3d-subshell has one electron with magnetic quantum number 0. Hence, a total number of electrons = 13.