There are a total of eight mainline companions that the player can recruit in the vanilla Fallout New Vegas experience. There are also DLC companions and temporary followers that one can acquire, but these are not considered mainline companions, and only the eight main companions count toward the 'the whole gang's here' achievement. We should also mention that it is possible to recruit three companions to follow you around at once. These are Rex, E-DE, and then any other humanoid companion.