Specifically, how can I find out the number of unpaired electrons in any ion?
First draw the electronic configuration for Iron.
Now since we are given with Iron(II) we take out two electrons from the outermost orbit so as to give the iron a +2 charge.
As evident from the configuration, in total there are 4 unpaired electrons.
Using the same logic you can count the number of unpaired electrons for any ion.
Not just any ion, but ground state ions. Following Aufbau and Pauli rules until you reach the transition metals.They fill up 3d orbitals after 4s orbitals are filled. But when they are ionized, their electrons fill only the 3d orbitals in ground state.
So Iron (II) ion will have 4 unpaired electrons.