Base: 200 Gamerscore / Max of 20 Achievements
DLC: Up to 50 Gamerscore + 5 Achievements per Quarter (3 Max)
Total: Up to 350 Gamerscore / 35 Achievements
The very first set of achievement rules for XBLA stood the longest: 200 Gamerscore and 12 Achievements. That was the only option for years. As Downloadable Content became more and more prevalent, we had an increase of 50 Gamerscore and up to 5 new achievements available. Then they made allowances for up to three DLC additions, garnering a new cap of 350 Gamerscore. Later, alongside the release of the Windows Phone platform, the base game could have up to 20 achievements instead of only 12, while the DLC rules remained the same. Starting soon, the policy is set to change once again.
Base: 400 Gamerscore / Max of 30 Achievements
DLC: Up to 100 Gamerscore + 8 Achievements per Quarter (4 Max)
Total: Up to 800 Gamerscore / 62 Achievements
The new rules as seen above are a gigantic step up. Doubling the base requirement of the Gamerscore itself, greatly increasing the number of available achievements and allowing for more DLC additions gives companies much more room to work with. These new rules give more opportunities for better progression of achievements through story and multiplayer, as well as allowing for more unique and fun achievements that we often see in retail games, but can be rare for an arcade. It also can lengthen the development cycle for a single game, allowing a company to offer up to a full year of DLC additions that can increase the game's Gamerscore.