Best way to know that is to monitor your server's http connections and configure accordingly.
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I would recommend you investigate LiteSpeed as an alternative to Apache if you are looking for such a high amount of traffic, you may even need multiple servers and load balancing depending on what these 50000+ users are doing.... maybe an idea also is to consider a grid/clustered provider like mosso/rackspace, could potentially save yourself time/money.
just change the MAXClients value and i think you need to increase the openfiledescriptors in Linux.
Litespeed is very expensive so i wouldn't use it.
If you don't mind NOT having cPanel, then get nginx or lighttpd because they will support 50,000 users out of the box.
first off, apache forks children so how high are you really going to set it anyway, 150, 250, 500.... its going to just eat up your ram and still be ineffective.
second, litespeed is cheap, the standard version is free I think which permits you i think 5 domains or something and 1 cpu (150 connections though), but most your load will be generated by external scripts anyway which will take advantage of all cores... A dual cpu enterprise license is maybe $40/month... aside from handling lots of connections quite well, lsapi is an also great.
litespeed will drop right in and integrate to your cpanel installation, and comes with quite amazing support.