The number of pages is limited by the amount of memory your computer has.
We use HylaFAX (linux base fax service) and there is a formula ( cann't not find it now) that acording to your memory size will tell you
how many pages your system can receive.
We dont have that problem because we use a dedicate machine to receive faxes and forward them as a pdf email attachement.
A simple solution will be to print the faxes inmetiatelly to a pdf printer (PDFcreator
http://sourceforge.net/projects/pdfcreator/) per example.
It has an "Auto Save" feature.
PDFcreator will save the "faxes" on your network drive as well
Kelloggs