Two, at this time (8 January 2011) there are two seasons and a third has been already recognised.
There are currently 4 complete seasons of Glee, but they have signed on to do an additional 2 seasons, meaning there will be 6 seasons so far. (As of summer 2013).
So far, there are 4 Adventure Time seasons.
6 Seasons
So far there are 4 seasons.
So far, FOX has ordered 3.
There are 3 seasons in Hell Girl so far.
10 seasons so far
There is 6 seasons of the hills so far
So far, 4.
It's unknown how many seasons they will have in the future, but so far there have been 3 seasons.
So far there are only two seasons and it is unknown if there is going to be a third
So far, only seasons 1 and 2 are out on DVD