Does anyone know if there is a upper limit for the maxium number of users that can read a single file from a NTFS share using Windows XP and Windows Server 2003?
The number of simultaneous connections to an SMB file server depends on available licenses, network and server capabilities, but there is no fixed upper limit.
For example, a modern Windows file server can easily serve thousands of clients. If only a single file is to be served, it will reside in memory after the first client has read it (and if memory is large enough). After that the number of clients is limited only by network throughput.