be there just to make up the number/pretend to be an expert
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 musicians very well.
A man named Nanguo heard about that and he managed to become a member of the band, even though he was 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 musicians 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 pretend to be a specialist. You can also hear people saying it about themselves to show their modesty.
其实“滥竽充数”的意思与“不懂装懂”是相近的，那在英语中就可以表达为“pretend to be an expert”，其中“pretend to be/do”的意思就是“假装是……”或者“假装做……”，而“expert”也就是“专家”的意思，所以“pretend to be an expert”也就是“假装自己是一个专家”的意思，那也就是“滥竽充数”啦~
有一本书的名字就叫做“Don't Pretend to be an Expert - Become One!”，那就可以翻译成“与其滥竽充数，不如学着吹芋”之类的。