Each DLC will raise the level cap by 5. So including Dead Money, if you've got them all, you'll have a cap of 50.”
He also addressed concerns that the level cap increase might make the game too easy:
And for you, I have good news. If you’re concerned that the new level cap increases will make the game too easy for you, I've got a couple things to note:
1) Congratulations on being hardcore
2) Old World Blues will offer a new trait (and respec your traits if you've already chosen them) that will allow you to cap your level at 30 (or current level if you’re already over 30).
Asked whether players will be able to continue playing after the main quest is finished, he again confirmed that the game will still end with the Second Battle of Hoover Dam:
Unrelated, wanted to note that none of the upcoming DLCs will allow you to play beyond the main quest (for the same reasons noted by the developers prior to the game's release).”