During the Warring States Period (475-221 BC), the King of Qi was very fond of listening to yu ensembles. He often assembled 300 yu players to perform grand music and treated his musicians very well.
A man named Nanguo heard about this and managed to join the band, even though he was not good at playing the instrument at all. Whenever the band performed for the king, Nanguo would stand in line and pretend to play. No one realized he was making no sound, so he enjoyed the same treatment as the other musicians.
When the king died, his son succeeded him. The new ruler also enjoyed yu music, but he preferred solos and ordered the musicians to play one by one. Consequently, Nanguo had to flee the palace.
The idiom "Be there just to make up the number" is used to mock someone who passes for a specialist. People may also use it about themselves to show modesty.