Far Cry 3 Walkthrough - Liberate all Outposts
There are 34 outposts in the game. They are marked by an icon on the map that includes a red flag. To liberate them, you have to kill all the enemies at the outposts. Once they've all been killed, the outpost has been successfully liberated.
You can liberate about half the outposts pretty early in the game. However, there is a southern chunk of the island that is inaccessible until you reach a specific point in the main storyline. This part of the island also has outposts to liberate, obviously.