There are 56 rackets to take over in the game. Take over all of them to earn yourself the achievement.
A lot of the shops you go into and extort the merchant of actually have another person you can extort as well. This person is a racket boss and is normally located in the basement or on the second/higher floor of the building. In hotels, the racket boss is normally near the top and in clubs and shops it is a mix of the basement and the second floor with some right on the roof.
If you feel that you should have the achievement and have searched high and low for the last racket to take over, remember that you can always check the stats in Tom's Report from the pause menu. You can view whether or not you have taken over the racket/front in all the Corleone controlled businesses.