According to Bohr, the maximum number of electrons can be accommodated in a principle quantum number n is $2{\text{n}}^2$. Here ‘n’ values starts from 1, 2, 3 and 4 or K, L, M and N.

$\text{K}-\text{shell}\to 1\text{s}=2\times {\left(1\right)}^2=2{\text{e}}^{-}$

$\text{L}-\text{shell}\to 2\text{s, }2\text{p}=2\times {\left(2\right)}^2=8{\text{e}}^{-}$

$\text{M}-\text{shell}\to 3\text{s, }3\text{p, }3\text{d}=3\times {\left(3\right)}^2=18{\text{e}}^{-}$

Then total number of electrons present in an atom is  $2+8+18=28$