I really doubt that 343i have plans for a 7, 8, and 9. Probably just buying the domains so others can't.
That being said, I doubt 6 is going to be the last game, either. Once they're done (or maybe even before they're done) with this upcoming trilogy, I'd expect to see more spin-offs, prequels, and side stories like all the stuff we got post-Halo 3, which I would be more than happy with.