Does Windows XP Professional have server restrictions, like how many concurrent TCP/IP connections are allowed?

Windows XP professional is a single user operating system. You can use it to make a server but it does not offer you any security. Yes, Windows has limits from 1024 to 5000 concurrent TCP/IP connections. For more information, you can check the following URL.

http://smallvoid.com/article/winnt-tcpip-max-limit.html