Which is the maximum number of Windows concurrent tcp/ip connections?

It could be because people are confusing the number of inbound SMB connections with the number of TCP/IP connections. SMB is used for file sharing and various other things and is limited to 10 connections in Windows Client OSes.

http://support.microsoft.com/kb/328459

The client versions of Windows are restricted in the number of half-open connections.

The 10 limit for XP is well known, Vista has other limits depending on the version (e.g. Home Basic has a limit of 2, Ultimate has 25).
You can Google "Patch for Event ID 4226" to alleviate these limits. Post Vista SP2 the behavior is configurable through a registry key.

Windows Server can handle several tens of thousands of concurrent inbound connections given enough resources.

Depending on your server version, you have to change some default parameters to allow these many connections to see this guide.

The author reports having seen 50.000+ concurrent connections to IIS6 on WS03SP1 x64 with 4GB RAM.