There is no limitation set by the driver to the number of addresses you can send at one time. It does, however, depend on how the printer handles the addresses and sends the information.
You should try using the Broadcast option next time you have to fax to a large group. This might help you alleviate the problem. After selecting "Fax" as the destination in the Print dialog, the next dialog that appears (Fax Destinations) has the "Broadcast using" popup field at the bottom where you can select "Computer" or "Printer". If "Computer" is selected, a separate fax job is created in PrintMonitor for each person in the group. If "Printer" is selected, then only one print job (for the entire group) is sent in PrintMonitor, and the printer handles sending the faxes to the individuals in the group.
Overall, this appears to be a VM (printer's Virtual Memory) constraint. Adding more VM in the printer, by adding more RAM, should increase the number of addresses per job.