Don't think anyone could give you a number because it's dependent upon factors un-known and unique to each server.
While number of users is something of which to be aware, it's more their usage of Moodle that's important.
Example ... a K12 school with 150 teachers and courses using the moodle while F-2-F with students (students number around 3000) means lots of traffic ... all day long (synchronously). That's not like a corp moodle (same number of folks/courses) who hit the moodle when they have time (asynchronously).
So increase the value gradually - 90 to 100 as an example.
Also, if your server is an all in one ... DB server on the same box as code (the moodle) remember it's a balancing act ... Apache cannot use all the memory ... MySQL DB server must have enough to run as well.
If you have some tool that shows what server is using/doing, access that tool and see how much memory is being used/free.