This is not possible. You cannot request the amount of messages sent in any way meaning you won't be able to access the messages sent before your bot joined at all. For messages sent after you can just increment an integer in the client.on('messageCreated', () => {}) listener.

If you really need the amount and you are savvy with python you may be able to create a separate program that opens discord, searches for messages sent by a specific user in the top right search bar, and returns the search results amount, but this would be a lot of work for something that seems inconsequential.