The Gamerscore (G) is a measure that corresponds to the number of Achievement points accumulated by a user with an Xbox Live profile. These Achievement points are awarded for the completion of game-specific challenges, such as beating a level or amassing a specified number of wins against other players in Xbox Live matches. Initially, retail Xbox 360 games offered up to 1,000G spread over a variable number of Achievements, while each Xbox Live Arcade title contained 12 achievements totaling 200G. These possible totals per game have been raised to 1250G with the one exception being Halo 3 which has 1750G and the Arcade games have been raised to 250G, see below.
On February 1, 2007, Microsoft announced on their Gamerscore Blog some new policies that developers must follow related to Gamerscore and Achievements in future releases. All games must have 1,000 Gamerscore points in the base game - the title can ship with fewer than 1,000 points, but anything added later must be free. Game developers also now have the option of adding up to 250 points via downloadable content (for a total of 1,250 points) - this content can be either free or paid. Xbox Live Arcade titles may add up to 50 points via downloadable content (for a total of 250 points).
Bit of a mouthful, but basically games can only be 1000 points except in a few rare cases. The highest is 1750 (Halo 3). Other than that, the max is 1250.