En replaces a phrase starting with de, including quantities. Quantes novel·les tens? En tinc tres. It also stands in for a whole topic: no en vull parlar. Catalan almost never leaves it out the way English drops of it.
Examples
- Quantes sèries has vist? En he vist dues.
- How many series have you watched? I have watched two.
- De cançons catalanes, en conec moltes.
- As for Catalan songs, I know a lot of them.
- No en vull parlar ara.
- I do not want to talk about it now.
The mistake to avoid
A number on its own is not enough: en tinc tres, never just "tinc tres".
Where this fits in
Aficions i temps lliure, Catalan A2: Talk about hobbies, likes and dislikes, and free-time plans.