The number of concurrent users allowed on a server is defined by the developer of the server. In the case of AppleTalk for VMS-based servers, there are ways to extend the number of users beyond the limitations set by the number of sockets per node.
There actually is nothing that limits an AFP server to having one connection per socket. AppleShare uses a one-client-per-socket method to simplify bookkeeping since the machines it is running on cannot support enough users to require multiple users per socket. The processing power and disk performanceusually provide the limitations of a server's capabilities.
The number of sockets allowed per node is not changing in AppleTalk Phase II. In both Phase I and Phase II, the socket number is represented as a byte, which gives 256 possible values: 128 of these are reserved, leaving 128 for dynamic socket allocation. Changing this field to a word size would require a major change to many of the routines and protocols of AppleTalk, beyond even the scope of the changes being implemented in AppleTalk Phase II.