Electron configurations

The electron configuration for atoms (M)

of first row transition elements other than Cu and Cr is 3dⁿ4s²
where n is the number of the column in d-block in which the metal appears.
of chromium (Cr) is 3d⁵ 4s¹
of copper (Cu) is 3d¹⁰ 4s¹

The first ionisation (M to M⁺) for ALL transition metal atoms results in loss of the 4s electron because this is in the highest energy orbital.

In the second ionisation (M⁺ to M²⁺) the highest energy electron lost is

a 4s electron for M⁺ other than Cr⁺ and Cu⁺.
a 3d electron for Cu⁺ and Cr⁺.

The third ionisation (M²⁺ to M³⁺) of all first row d-block metals involves removal of a d-electron.

This means that for first row d-block metals (column number 3 Sc Ti V Cr Mn Fe Co Ni Cu Zn column number 12)

with the exception of scandium, all M³⁺ ions have incomplete d-subshells.
with the exception of zinc, all M²⁺ ions have incomplete d-subshells.

Copper(I) and zinc(II) have full 3d-subshells.
Scandium(III) has no d electrons.

The number of d electrons in a d-block monatomic metal ion equals the number of the column in which the element appears in the periodic table minus the charge.

Zn is in column 12 therefore Zn²⁺ has 10 d-electrons. Sc is in column 3; therefore Sc³⁺ has no d electrons.
If the 1-18 numbering is used for the periodic table the number of the column is equal to the group number.