Let \(A = \{n \in \N \st 20 \le n \lt 50\}\) and \(B = \{n \in \N \st 10 \lt n \le 30\}\text{.}\) Suppose \(C\) is a set such that \(C \subseteq A\) and \(C \subseteq B\text{.}\) What is the largest possible cardinality of \(C\text{?}\)