should be at least several missions per planet and there are 4 confirmed and more expected. including strikes, raids and explore, there's already much more than Bungie's traditionally done for campaigns. PvP alone sustains people for years, and they have several expansions planned.
Im not worried about content. It just needs to be engaging and replayable.
Personally, I feel there's some unspoken expectation for
absurd amounts of content in Destiny. Some of it can be blamed on Bungie somewhat, given that they did initially open up the Destiny reveal with a ton of location concept art.
IMO, if I take the current content of the beta, add an extra 20-30% to account for the places currently being blocked off ( King's Watch, etc), and then quadruple that amount across the 5 confirmed locations ( Earth, Moon, Reef, Venus, Mars)... That is not a small amount of content by any means. And above that, there's PvP (Crucible) and the Vanguard as well. And we can't be absolutely certain that there aren't stuff that's still being somewhat kept under wraps.
IMO, even looking at the minimal expectations of Destiny, I don't think it'll be short of content by any reasonable stretch of the means. Maybe compared to Skyrim or GTAV, but I'm not going to fault Bungie too much if there isn't insane amount of stuff for the first-game.