Sum of Digits - Base Case

The base case is when $n < 10$, meaning $n$ is a single digit. In this case, the sum is just $n$ itself. For example, if $n = 7$, the sum of digits is $7$. You do not need recursion here.

This stopping condition keeps you do not try to break down a single digit further. Once you have reduced the number to one digit through repeated division by $10$, you are done. No more digits to peel off. That's your signal to stop.