when we use "en" ex:il y en a qui exagérent

Bonjour Abderrahmane !

This case is tricky as we omit words in both English and French!
"Il y EN a qui exagèrent !" = "There are [some] OF THEM who are pushing it!"

In French, "en" is used to replace "of them" with quantities, but in this case, we don't express the quantity "some".
We should say "Il y en a [certains] qui exagèrent !", but the expression "Il y en a qui" is very idiomatic and colloquial.

Here is a link to the lesson on "en" :
En with quantities = Of them (French Adverbial Pronouns)

I hope that's helpful!
À bientôt !