As supervisor of the toll booth, Levi apparently has the use of a large house in which not only Jesus and his disciples can recline to eat, but also many tax collectors and sinners. With a small comment, Mark suggests how there could have been many guests: for there were many who followed him.
This can mean that there were many tax collectors and sinners in the crowd of followers, but it can also mean that it was precisely the people who followed Jesus when he entered Levi’s house, while others (scribes and Pharisees) did not enter but look for accommodation elsewhere. The latter is more likely considering what follows. By calling Levi, the tax collector, and becoming a guest in his house, Jesus creates a remarkable situation: those who together formed a crowd during the day, now break up into people who join in the meal and people who do not want to do that. This creates the situation and raises a question that Jesus wants to answer.
15 And as he reclined at table in his house, many tax collectors and sinners were reclining with Jesus and his disciples, for there were many who followed him.