We know that as per Bohr Model.
 Electrons move around Nucleus of an atom in discrete orbits .
 These are also called energy levels or shells .
What is the number of Electrons in a Shell?
 The distribution of the electrons in the energy shells is known as electronic configuration .
 This is based on the BohrBury scheme according to which, the maximum number of electrons that can be present in a particular energy shell of an atom is given by 2n 2 , where “n” is the number of energy shells.
 The energy shells K, L, M, N are the first four energy shells given by Bohr .
 So, the maximum number of electrons which can be accommodate in each shell is given as:
Electron Shell 
Maximum Capacity 
K shell 
2 electron 
L shell 
8 electron 
M shell 
18 electron 
N shell 
32 electron 
Note:  The order of energy of these energy shells starting form nucleus is K < L < M < N.
How is Maximum Number of Electrons in a shell Calculated
It is calculated by using formula, 2n 2
Example: 
 For K shell, n = 1
So number of Electrons = 2n 2 = 2 x 1 2 = 2
 For L shell, n = 2
So number of Electrons = 2n 2 = 2 x 2 2 = 2 x 4 = 8
 For M shell, n = 3
So number of Electrons = 2n 2 = 2 x 3 2 = 2 x 9 = 18
 For N shell, n = 4
So number of Electrons = 2n 2 = 2 x 4 2 = 2 x 16 = 32
Note: Electrons are first filled in the innermost shell first.
Then, it is filled in the outer shell.
Example:

Helium has 2 Electrons
All these are in K Shell (as K Shell can hold max 2 electrons)

Lithium has 3 Electrons
2 of these are held in K Shell (as K Shell can hold max 2 electrons)
Remaining 1 are held in L Shell

Sodium has 11 Electrons
2 of this are held in K Shell (as K Shell can hold max 2 electrons)
8 of this are held in L Shell (as L Shell can hold max 8 electrons)
Remaining 1 is held in M Shell
Examples 
In text Question  Page 50 Q1, Q2
NCERT Back Exercise  Q6, Q9, Q12