And why would you want to know when MaxClients is reached? It's not an error, it has no implications for security, and it could happen dozens of times a day depending on your configuration. Raising the value of MaxClients whenever you get the alert is also a recipe for disaster. See
this thread.
You could write a script that uses "tail" to grab the last X lines of Apache's error log, "grep" to find the MaxClients entry, and "wc -l" to check if there are any matching lines. If a matching line is found, use the "mail" program to send an e-mail. You could put the script in root's crontab and run it every Y minutes.