I think that this is the relevant passage from the winsock FAQ

4.9 - What are the "64 sockets" limitations?

There are two limitations in Winsock where you're limited to 64 socket
+s.

The Win32 event mechanism (e.g. WaitForMultipleObjects()) can only wai
+t on 64 event objects at a time. Winsock 2 provides the WSAEventSelec
+t() function which lets you use Win32's event mechanism to wait for e
+vents on sockets. Because it uses Win32's event mechanism, you can on
+ly wait for events on 64 sockets at a time. If you want to wait on mo
+re than 64 Winsock event objects at a time, you need to use multiple 
+threads, each waiting on no more than 64 of the sockets.

The select() function is also limited in certain situations to waiting
+ on 64 sockets at a time. The FD_SETSIZE constant defined in winsock.
+h determines the size of the fd_set structures you pass to select(). 
+It's defined by default to 64. You can define this constant to a high
+er value before you #include winsock.h, and this will override the de
+fault value. Unfortunately, at least one non-Microsoft Winsock stack 
+and some Layered Service Providers assume the default of 64; they wil
+l ignore sockets beyond the 64th in larger fd_sets.

You can write a test program to try this on the systems you plan on su
+pporting, to see if they are not limited. If they are, you can get ar
+ound this with threads, just as you would with event objects.
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