The number of electrons that the d orbitals are capable of intaking or releasing beyond the number of 4s electrons represents the valency of the d block elements. However, the d block elements may have variable valencies. 

Example, Iron has atomic number 26.

Electronic configuration - 1s2, 2s2, 2p6, 3s2, 3d6, 4s2

Here, the 4s electrons are removed first. In that case, the valency of Fe = 2+.

Beyond this, if now 1 electron form 3d orbital is also removed so that it obtains the half filled orbital stability, then we get valency of Fe = 3+.