[charliesome]

PIDs will recycle rather than exhausting. As an aside, that's also the reason why it's unsafe to use a PID from any process other than the parent process. As soon as the parent calls wait(2), the PID is released and could be reused for a fresh process at any moment.

[JJMcJ]

The only way to run out of PIDs is not wait() properly, leading to zombie processes, or else some CGI processes time out and don't exit, so never reaped by a wait().