i have big project, but 85000 user is not always online, but sometimes happend when examp test has been started
I believe you will need a distributed infrastructure with more than one server.
I'm studying the use of openshift with moodle containers to handle a lot of hits.
Another thing you should worry about is the cache server.
You can try somethink like this: https://severalnines.com/blog/clustering-moodle-multiple-servers-high-availability-and-scalability