滥竽充数

During the Warring States Period (475-221BC), the King of the State of Qi was very fond of listening to yu ensembles. He often got together 300 yu players to form a grand music. The king treated his musician very well. A man named Nanguo heard about that and he managed to become a member of the band, even though he wan not good at playing the instrument at all. Whenever the band played for the king, Nanguo just stood in the line and pretended to play. Nobody realized he was making no sound at all. As a result, he enjoyed his treatment just as the other musician did. When the king died, his son became the new ruler who also liked the music played on the yu. However, he preferred solos so that he ordered the musicians to play the yu one by one. Therefore, Nanguo had to run out of the palace.

The idiom "Be there just to make up the number" is used to mock someone who passes for a specialist. You can also hear people saying it about themselves to show their modesty.