The number above comes from the latest edition of Edge Magazine, issue 310, where a cover story discussed many of the details being implemented in Destiny 2. It doesn't seem like players will run the risk of growing tired of the numerous quests either.
In addition to having more than 80 missions to take on, it was also reported that each one of them would be quite substantial in more ways than one. Length, difficulty level, story engagement, and the possibility of worthwhile rewards are all huge factors that were taken into consideration when constructing the many missions.
If all of this content may seem like a bit much at first glance, you're in the same situation that the developers were when they added all of this content at first. With so many missions and other related content being included, the article in the magazine also indicated that Bungie realized at one point that they had much more content than they had progression, something that they've since changed to balance the two motivators out.
Other details revealed in the article discussed areas of the game such as combat elements and game modes. Energy weapons were one point of discussions where it was revealed that the games beta did not include the final version of Energy weapons as far as their interactions with AI are concerned. When used against these AI enemies who have elemental shields, an explosion will actually occur when the shield is depleted, blasting any additional enemies that happen to be nearby.
Time limits will also be put in place in the Nightfall Strikes, so players won't have quite as much free time as they'd hope for when developing their strategies. The time limits will vary throughout the Strikes, but one example that was revealed was a 13-minute limit.
Several other details were discussed that were unable to be reported after Edge Magazine was asked to keep them on the down low, but the majority of the discussions in the article seemed to paint a positive picture for Destiny 2.
[Edge Magazine via Reddit]