Whatever proves to be a good measure of "server is busy". If requests are relatively constant in resource cost, then you might have a hard ceiling of "N max simultaneous requests" and a semaphore would be a good fit (and you'd need to pre-fork upto N+1 children, of course).

But you might have a different "busy" metric you want to use like "system load average" or "idle CPU percentage" or "virtual memory in use" or whatever.

But, even if you do implement some "busy" metric other than "N simultaneous requests", you'll probably still have some "max N children" configuration and so a N-1 semaphore is probably still a good idea.