The OPC specification does not define, and does not expect any aritifical limit (besides the fact that the item count is a 32-bit DWORD). The OPC server may have internal limits given by the resources available. However, the limit should not be expressed in number of items per group, because the OPC client has no way of knowing the limit, and if the client decided to overcome the limitation by creating more OPC groups, the demand on the OPC server would be even higher. If the OPC server must limit the number of items it can handle, it should do so using a limit that is server-wide, or per-client; but not per OPC group.
We have been notified of one OPC server that limits the number of items in an OPC group (Yokogawa?); there may be more, but it is certainly not typical. Such behavior poses a big interoperability issue.