What number is Boeing going to use after 797 ?
They might not last long enough to find out.
666............................
Their originally proposed 797 (NMA - New Midsized Aircraft) - intended as a 757 replacement - is exactly what the market is clamoring for right now.
Boeing's management geniuses decided not to build it, guessing that the proposed and as yet uncertified 737 MAX 10 would be the cheaper and better option.
Airbus has already gobbled up the entire mid-market with its newer A321 Neo and XLR variants. Developing a 797 at this point would do Boeing little good, unless it is some kind of radical new design that knocks the whole industry on its ear.
I'll leave it as an exercise to the reader to guess how likely that is. :)