Hi Rob,
I don't think I've done it myself, but I thing Graham Smith has done a multi-file-form before.
I don't think there's any restriction on the number of file upload fields on a page.
So let's say you have 10 fields on the page. One idea is to make them all hidden, and then unhide them dynamically as the user enters files. So rather than show all 10 to start with you just show 1. Then if he fills that in, you unhide number 2 and so on.
Cheers
Bruce