You simply need to hire a group of courtesans 34 times (150 florins x 34 = 5100 florins total). The courtesans respawn if you leave the city, so if you want to "boost" this, hit the known courtesan locations, leave the city, return immediately, and repeat. There are two groups behind Paola's house, and a group near some of the gates into Firenze/Florence.
Note: You can obtain your count of courtesans hired by going into the start menu, selecting Options, then selecting Stats, then Social.