The atomic number of iron, Fe, is 26. This means it has 26 electrons.
Now, as we write the configuration, we come up to electronic subshell 3p, which is completely filled. So the configuration up till this point is identical to Argon, so we may write:
[Ar], 3d, 4s
After the electrons in the Ar configuration are accommodated, there are 8 electrons left for the 3d and 4s shells. The 4s subshell, when empty, is at a lower level than the 3d level, so it fills first. Two electrons are placed here. Therefore, 3d has 6 electrons in Fe.
