Identify the simple subject in the sentence "There are too many people on this elevator."

The subject is who performs the action indicated by the verb in the sentence. So you can find the subject by asking who to the verb. In this case the question is who are on this elevator, whose answer is too many people. That is the complete subjec. The simple subject is the main word of the subject; it does not include the modifiers. So the simple subject is people.