There are a total of 27 green Safkas in the game. There is one on each of the nine levels in each of the three chapters. There are no Safkas on the bonus levels, the eggilogue or the prologue. You do not need to beat the levels to get credit for finding the Safkas. The achievement will unlock as soon as you reach the final green Safka (again, you don't need to beat the level).
You can see which Safkas you've gotten in the level selector. Moving Ilo and Milo to each level shows you which Safkas you've found in the lower left corner (next to the number of cubes, if you've beaten the level)
Also, video walkthroughs for all levels, including the location of all safkas, can be found here:
http://www.youtube.com/user/SwamiMacGyver