As of June 2015, 25 seasons have aired.
As of November 2014, that would be 22.
What exactly are you asking? You want to know the sources that indicate how many seasons have aired? 28 seasons have aired, with the show being renewed through its 30th... as for sources that state that, there are several to choose from. Wikipedia. FOX. SimpsonsWiki.
While technically no one can know how many episode were literally made in 2009, 20 new episodes aired in the calendar year.
234 episodes aired during that decade.
At the end of 2012, 516 episodes had aired.
22
21 Seasons have aired on CBS. The 22nd is currently underway.
To answer your question a total of 4 seasons have aired and season 5 started last weekend.
180 episodes aired over 9 seasons.
As of the 2013-2014 season, 24
Five
429 at the start of 2009449 by the end of 2009