In a section detailing how to install Halo: Reach to the Xbox 360 HDD, Bungie included some screenshots.
One of which clearly shows a possible 1250 Achievement Points and 59 total achievements for the title. Halo: Reach currently caps at the requisite 1000 points through 49 achievements.
A Microsoft spokesperson has confirmed that Halo: Reach will--unsurprisingly--get DLC, releasing the following comment to Shacknews:
We plan to extend the online experience of "Halo: Reach" with a robust lineup of Add-on content for the most popular game on Xbox LIVE today. While we don't have any specifics to share at this time, we look forward to revealing additional details on new Add-on content for "Halo: Reach" in the near future.
Halo 3's paid DLC, which was comprised entirely of multiplayer map packs, all added new achievements along with new arenas. It's likely that any DLC for Reach would include more multiplayer maps, but it is uncertain if other areas of the game--campaign or Firefight--will see new content.