How come some elements fit up to 8 electrons in their 3rd shell and others can fit up to 18?

electrons go into the lowest energy level available to them. It just happens that after putting 8 electrons in the 3rd shell, it takes less energy to start putting electrons in the 4th shell. That's why calcium has only 8 electrons in its 3rd shell and 2 in its 4th shell.

After putting 2 electrons in the 4th shell it takes less energy to put them in the 3rd shell again. So any atom that already has 2 4th shell electrons, like copper, will get more electrons in its 3rd shell.

Nickel has 16 electrons in its 3rd shell and 2 in its 4th shell because that is the lowest energy configuration.