Hi pmarie,
we have an installation with 5 MVs (on VMware) as fronts that support up to 1000 concurrent users. To do this, we have a balancer F5 that distributes the load between said fronts. Unless some expert of the forum in the matter says otherwise, the main limitations will be the type of Web Server deployed (eg Apache, Nginx, ....) and the way PHP is used (FastCGI Process, ....) which will determine the RAM requirements for each virtual machine by number of users. To improve the performance and therefore the number of concurrent users who can access your infrastructure, another point to consider is the optimization of the Database (both at the level of performance improvement by HW -disks SSD, RAM, ...- as to the level of improvement in the use of indexes, optimization of ILIAS queries, ....).
Greetings.