Okay so start out with the electron configuration which is [Ar] 4s2 3d8 . Now you see the s orbitals are filled up so you move on to the d orbitals which can have a maximum of 10 electrons. But in Ni, 8 of them are filled. So 2 electrons are unpaired. Hopefully I helped!
The last electrons in a nickel atoms are the 8 3d electrons. If you put those 8 electrons into the five 3d orbitals, you put one in each orbital and then you start pairing them up (the 3 left). So you have 3 3d orbitals with a pair of electrrons in them and 2 3d orbitals with single electrons in them.