When we send ajax request to some php file via post, we get the variables in php file from $_POST, and then send response as a text or html or JSON or whatever with echo or print.
So, what if a lot of such ajax requests come simultaneously? what server does in this case? does it process all of the requests (every request)?
and does it make copies of this php file for each request or does it work with single php file by queueing the requests?

Taken from: http://www.crucialp.com/resources/tutorials/server-administration/optimize-tweak-high-traffic-servers-apache-load.php

"Limit total processes with MaxClients:
... Use trial & error to figure out how many Apache processes your server can handle, and set this value in MaxClients. Note: the Apache docs on this are misleading - if this limit is reached, clients are not "locked out", they are simply queued, and their access slows. Based on the value of MaxClients, you can estimate the values you need for StartServers, MinSpareServers, & MaxSpareServers."