Let the total number of boys in the row be $N$.
Sudhanshu is $12^{th}$ from the left.
When he moves $4$ places to the right, his new position from the left becomes:
$12 + 4 = 16$
After shifting, he is also $18^{th}$ from the right.
The total number of boys in the row is given by:
$N = \text{Position from left} + \text{Position from right} - 1$
Substituting values:
$N = 16 + 18 - 1$
$N = 33$
Thus, the total number of boys in the row is:
$\boxed{33}$