There is no limit on the number of faxes you can have stored/retained at a point in time. You can always view and search for any and all faxes via the Documents menu.
I wonder if it's possible that there is an issue with our inbox functionality where you're unable to page through all the faxes you presently have. Could you open up a support ticket to track that down? In the meanwhile, the Documents menu should provide you access to all faxes.