Numbering of family page files is dependent on the family record number (MRIN). Each family stored in a GEDCOM file is assigned a MRIN. How these numbers are assigned depends on the software that you are using to create the GEDCOM. Normally MRIN #1 would the first family you created in your database, but some genealogy software programs are not consistent with maintaining MRINs. Therefore there may or may not be a fam001.htm.
On the table contents page created by UncleGED (index.htm) there is a link labeled "Family Pages". This link points to the lowest numbered family (MRIN) found in your GEDCOM.
I tried the following experiment involving MRINs and the numbering of family pages. I built a test database in both FTW and PAF. I added individuals/families to both databases in exactly the same order and at various steps I exported both files out to a GEDCOM and compared the two. In no case did either of the programs change the MRINs. I tried all sorts of additions to both files and could not get them to change the MRINs.
However I did find a case where FTW will change the MRIN. The version of FTW that I have (7.0) basically only gives me two options for exporting out to GEDCOM: export everything or export one particular branch. With PAF I can use filters to select individuals based on certain criteria. The filter that I used for my webpages is only if the birth date is earlier than 1910.
I did an full export from FTW and compared that with a partial (single branch) export from FTW and sure enough the MRINs were changed. I also noticed that from within FTW I can't even see what the RINs/MRINs are. Whereas PAF both are displayed all over the place. I think that basically the architecture of the FTW database does not care about MRINs, whereas PAF is closely tied to maintaining the MRINs. So there's really no blame to FTW; it's just built differently.
Can the MRIN be changed? No, neither PAF, FTW, nor Legacy allows you to do this.
The list report file will allow you track which HTML file an individual appears in. For each individual this will list either display the individual's parent's family file or the individual's family file depending on the individual's recorded marriage status.