As many of you already know Starcraft 2 is releasing with just one races campaign. There is a FAQ on the official blizzard forums located here

How long is each of the campaigns?
StarCraft II’s terran campaign will consist of approximately 26 to 30 missions, and each expansion set will include a similar number of missions. This means that the complete StarCraft II Trilogy will include as many as 90 single-player missions. This allows us to create a truly epic story experience with a great variety of unique missions and gameplay types.


Well that at least confirms 2 expansions for the game.