As your server gets busier, Apache will want to start more processes to handle the requests. In turn, MySQL will want to start more processes to deal with incoming database requests.
It sounds like you really need to fine-tune a few things. Depending on how you use Moodle and your number of concurrent users, you'll want to tune:
- Your maximum PHP script execution time.
- Your maximum PHP script memory usage.
- The number of processes Apache is starting (this is an art and a science, and depends on the memory your system has). Don't forget MaxSpareServers, MinSpareServers and StartServers too!
- Your max_connections in MySQL (the total number of MySQL threads dealing with requests)
- Your thread_cache in MySQL (the number of processes MySQL keeps around even if it's doing nothing)