Christian tradition tells us that there were exactly twelve disciples, no more, no less. Unless, of course, you read John's Gospel. Aside from "the twelve" (6:67) who are always hanging around, there seem to be plenty of other men and women who make and appearance.
In addition to Peter, the Beloved Disciple, and Mary Magdalene, John's Gospel names several disciples specifically: Andrew, Nathanael, Phillip, and Thomas. Usually, the spare disciples are just milling around asking questions and generally being confused about what Jesus means by things. It's quite helpful, actually, because they provide an avenue for Jesus to answer questions and keep talking. Oh, Jesus, won't you tell us more about the vine and the branches? We have no clue what you're talking about.
Let's look at an example:
Before Jesus multiplies the loaves and fishes, he asks his disciples how they're going to get more food. Philip tells him, "Six months' wages would not buy enough bread for each of them to get a little" (6:7). Andrew offers, "There is a boy here who has five barley loaves and two fish. But what are they among so many people?" (6:9). It's only once the disciples have registered their doubts—and proven themselves to be a little thick—that Jesus rolls up his sleeves and gets to work proving them all wrong.
Don't worry, ladies, there's room for you in the 1st century, too. In addition to Mary Magdalene, we meet Mary (the wife of a guy named Clopas), Jesus's aunt, and Jesus's mother, named—wait for it—Mary. Martha, Lazarus's sister, is definitely a follower of Jesus, too.
There really is something about Mary. The name was by far the most common for Jewish girls in the first century. Why? Maybe because it was also Moses's sister's name—Miriam.
Not everyone is cut out for a life following Jesus.
In addition to Judas, who flat-out betrays Jesus, there are other disciples who leave him behind after they hear his teaching about eating flesh and drinking blood. And we have to admit, it's questionable preaching strategy.
The disciples that remained would have almost certainly put themselves in danger by sticking with Jesus. Think about it: they're following around a guy who enrages the folks in positions of power and who people alternately want to worship and kill. You've got to be pretty tough to stick that one out.