​If maximum no. of electrons can be filled in M-shell is 18,then why electronic configuration of calcium is  2,8,8,2   ,knowing that outermost shell can have maximum 8 electrons and no new shell is filled unless the inner ones are filled completely,but here it only has 8 electrons in M-shell?

It is due to octate rule The octet rule refers to the tendency of atoms to prefer to have eight electrons in the valence shell. When atoms have fewer than eight electrons, they tend to react and form more stable compounds. When discussing the octet rule, we do not consider d or f electrons. Only the s and p electrons are involved in the octet rule, making it useful for the main group elements