jmc

Suppose we define $\ell (n)$ as follows: If $n$ is an integer from $0$ to $20,$ inclusive, then $\ell (n)$ is the number of letters in the English spelling of the number $n;$ otherwise, $\ell (n)$ is undefined. For example, $\ell (11)=6,$ because "eleven" has six letters, but $\ell (23)$ is undefined, because $23$ is not an integer from $0$ to $20.$

How many numbers are in the domain of $\ell (n)$ but not the range of $\ell (n)?$