In a policy decision, that Microsoft claims is to prevent players from being overwhelmed by things to accomplish in a game, they will be setting a limit on the number of post-launch achievement updates a developer will be able to add into their game.
With the Xbox One, developers will be able to add in-game achievements, post-launch, without having to bundle them into a downloadable content package (DLC). While this is very helpful to the developers, Microsoft does not want players who are trying to attain a 100% Gamerscore to be constantly overwhelmed with the addition of new achievements pouring in too frequently.
According to Chad Gibson, Microsoft's Xbox Live Principal Program Manager, they are "super sensitive to people who are worried about wanting to get all of the Gamerscore, and also added, "We still have policies for developers to make sure that things don't get out of control. Like, we do not want a game offering 10,000 Gamerscore every day. We still have policies, so that user experiences are rational - users have an opportunity to complete everything without feeling constantly overwhelmed."
While getting all of the available achievements in a game should be challenging, it would be pretty disheartening to buy a game a year after launch and find that there were hundreds of thousands of achievements needed to hit that all coveted 100% Gamerscore.