Server will crash under high load only if you run out of memory and there is too much disk thrashing.
I was mentioning in an earlier post that I have hosted on a Virtualmin server lots of WordPress website for a computer training institute for students to use while they learn website design.
The load on the server for most hours of the day is minimal but for the two hours in a day when the course is being conducted, over 100 students log in simultaneously and carry out the same operations as the trainer instructs the group - hence at such times there is lots on activity on the server in terms of memory usage, web and db server access and disk i/o.
How we were able to avoid slowdown is to insure that there is no disk thrashing and memory is released quicker than usual by php-fpm.
Hope this helps you with tweaking your server for better performance under high load.