Since a map can only have at most one key, count will essentially stop after one element has been found. However, in view of more general containers such as multimaps and multisets, find is strictly better if you only care whether some element with this key exists, since it can really stop once the first matching element has been found.
In general, both count and find will use the container-specific lookup methods (tree traversal or hash table lookup), which are always fairly efficient. It’s just that count has to continue iterating until the end of the equal-range, whereas find does not. Moreover, your code should document intent, so if you want to find something, use find.