Why does Ca does not go with the 2n2 rule?

But, according to the 2n2 rule, the M-shell can hold upto 18 e-. So, doesn't this mean that the e- configuration of Ca can also be written as 2,8,9,1 or say, 2,8,7,3 or something..
Really the only way to know is through experiment or possibly very high level calculations that include relativity and quantum electrodynamics. As atomic number increases, the problem becomes more and more complex. It isn't necessarily true that one configuration accurately describes the ground state.

For example for Lawrencium (element 103), for which no experimental determination has been made, you will see some periodic tables that say it has a 7p electron (8 electrons in the P shell and 3 electrons in the Q shell) and some that say it has a 6d electron (9 electrons in the P shell and 2 electrons in the Q shell).