The query itself can select any number of rows from the 'employees' table, as there is no restriction on the number of rows returned by a SELECT statement. However, for the WHERE clause to work correctly with the '=' operator, the subquery must return only zero or one row. This is because the '=' operator is used for scalar comparison, which means it can only compare a single value on the left with a single value on the right. If the subquery returns more than one row, it would result in an error or undefined behavior.