I run XAMPP on XP Pro but simply for development (I use hosting services for production sites.) Your question interested me so
I found http://support.microsoft.com/default.aspx/kb/314882
Inbound connections limit in Windows XP and it appears that the 10 connection limit is enforced on some capability but "The TCP connection limit is not enforced, but it may be bound by legal agreement to not permit more than 10 clients." From aspfaq.com (re bypassing the 10 connection limit enforced by IIS) " so it is possible to use a different web server to bypass the limitation — but you will still be violating your license agreement."
I know I am not much help but as I have it.
On your comments - 10 user limit on file share - Yes
Will Apache limit web access to 10 connections - No and according to Microsoft's kb article neither will XP Pro (but wheteher you XP licence allows you to is not clear.)