There are a total of eleven buddies. Six in the northern territory and five in the southern territory. You MUST rescue the six in the north before you escape to the southern territory because they will no longer be available once you leave.
Three of the buddies in the north are resuced as part of the main storyline. Two others can be found by exploring the areas around Pala - the lumber yard, the cock fights, fresh fish, the slaughter house, etc. You're looking for a building that you can actually enter through a closed door. Once you enter the building you should hear somebody shouting. Look for an interior door that you can unlock by sliding a bolt open. You'll find the buddy inside that room. The sixth buddy is the one that's easiest to miss. He or she will be next to a crashed plane in the northern middle section of the map. It's due south of the Jackal tape in that area, due west of a safe house, and due north of Shwasana.
In the southern territory, four of the buddies are unlocked through main story missions and the fifth can be found in one of the buildings in the Brewery. The achievement will unlock once the final buddy is rescued.