There are 84 businesses to extort in the game in all of the districts of the city. Take control of all of them for the achievement.
The higher your respect level, the easier it is to extort the merchant. Every merchant has a weak point which makes them reach their breaking point quicker allowing you to complete the extortion.
The Tattaglia family businesses are easiest to extort with the Cuneo and Stracci on pretty similar levels. Barzini businesses demand a higher respect level. As I mentioned before, having a higher respect level makes it a lot easier to extort the merchants.