Good afternoon and thank you for your question. As far as I can see, even in the output on the article under https://learn.microsoft.com/en-us/graph/api/user-findmeetingtimes?view=graph-rest-1.0&tabs=http, it only does provide up to 5.
If you need further verification on this, I will highly recommend creating a ticket so that our team can verify it and update the documentation if necessary.
If I was able to answer your question, please upvote and accept it as an answer.
Have a great day ahead! 😊