The number of elements of a finite set $A$ is written as $n(A)$ or $\vert A \vert$. It is also called the cardinality of the set $A$.
$$n(\{A,B,I,L\}) = 4$$
A subset $A$ of a finite set $B$ is finite and has fewer elements than the set $B$. This can be written as $$n(A)\le n(B).$$