Additionally, depending on the setup of your RP, you might not need a ton of people for the RP to be stable and keep running. Plus, once you've got it started, who's to say more people won't want to jump on board?
Of course, whether or not newcomers can join beyond that point depends on the setup of the RP and you're GMing style, but yeah, my point is, you don't have to worry about numbers so much. If you only get a few people and advertising/recruiting isn't working for you, then just focus on making the RP work with the members you have. Sometimes a tight-knit group of 3-4 players is a lot better than a larger group within which half the players will lose interest and leave, forcing you to work around the problems caused by their sudden absences.