So how many map packs do you think will release for Reach?
We had four in Halo 2 (Bonus, Killtacular, Maptacular, Blastacular).
Three/four in Halo 3, depending on how you look at it (Heroic, Legendary, Mythic I, Mythic II).
343i/CA are lending a hand with map development in a stronger capacity than the Halo 2 days (CA created the Blastacular Map pack).
Bungie will develop one more map pack, and 343i will make another.
But 343i's next project might take precedence over Reach when the fourth map pack hits. So I'd say four.