There are 5 seasons and 65 episodes of Merlin (TV Episode) from 2008 to 2012.
So that, Merlin(TV Series) have 5 seasons :
1. Series 1 (2008)
2. Series 2 (2009)
3. Series 3 (2010)
4. Series 4 (2011)
5. Series 5 (2012)
See more details here : TV.com