Chemistry double checking

Calcium

It has 2 electrons in its outer shell.
Using 2n^2
Current electrons on its 4th shell
Electron configuration ends with 4s2

This bit I’m not sure
Has 3 full shells? - 2+8+18= 28 electrons

It will not have 18 electrons in its 3rd shell- only 8. The 3d subshell is empty in Ca. When the 3p shell is filled, the next one to be filled is 4s.
When 4s is full, then the 3d shell is filled.
So Ca is 1s2 2s2 2p6 3s2 3p6 4s2
the next element in the table is Scandium Sc 1s2 2s2 2p6 3s2 3p6 4s2 3d1 or 1s2 2s2 2p6 3s2 3p6 3d1 4s2 if you prefer

Oh wait so is 2n^2 misleading to find the number of electrons?

I am not familiar with that formula .. it clearly gives the wrong answer here...so it is best to learn the order in which subshells fill up.

If electronic configuration ends in 4s, its in period 4 of the periodic table, it has 4 electron shells or energy levels. It is also in the s block, so its in groups 1 or 2, the alkali or alkaline earth metals, 4s1 would be potassium, as its in period 4 and group 1, 4s2 would be calcium as its in period 4 group 2 (alkaline earth metals).
2n^2 tells you the number of electrons in the shell or energy level, if n=4, for period 4 elements then you can have the following sub-levels in the 4th shell: 4s, 4p, 4d, 4f, and since
s sub-level holds 2e
p sub-level holds 6e
d sub-level holds 10e
f sub-level holds 14e
total of 32 e in the 4th shell or energy level, which is 2n^2, or 2 x (4^2) = 32