It's always worthwhile to keep in mind that MaxRequestWorkers is the actual amount of workers that can use up to the memory_limit you have set in your php.ini file. The default is 128M (minimum 96M, 128M recommended, increase if you have issues or large moodle instances/backups). If you have a server with 16GB of RAM, you should always set aside some for overhead of the system (usually 2GB is common) and then do your calculation.
(16GB-2GB)/128M = 112
This number, 112, is the amount of workers that your system can handle using the max amount of memory on your system before it starts to swap (page) to disk and your performance will tank. That doesn't mean you can't set it higher but depending on your scenario it is usually better to keep in memory for your applications so the performance is consistent. Some people use the average apache process memory usage to calculate the amount of request workers they should have but if you go this route you will need to be careful about with performance issues when you run at capacity. Only you truly know your workload so adjust to what you see is best for your environment.