Jesus had a large group of followers during his ministry, at one time dispatching 70 of them, two-by-two, into the countryside to preach.
However, his most trusted and tight-knit group of followers were just twelve in number, and were dubbed by him, "apostles."
What we know about most of these men comes exclusively from the gospels, (and in may cases, that is scant). For several of them, though, there is information available from the writings of early Christians and non-biblical scholars. These writings serve as the basis for a variety of traditions about these men, what they did, and sometimes how their lives may have ended.