My understanding is that en in this context as a pronoun, is used to mean some of or a bit of. J'en ai / veux. = I have / want some. En as-tu? = Have you got a bit? Je n'en veux pas. = I don't want some. It can be confusing as they also say J'en ai deux. = I have two. but this seems to use in the context of when there are a group of items and you take two of them. Let's say you have bought a bag of apples and you ask some someone if they have taken one, they could answer Non, j'en ai deux.
Absolutely. I used "of them" in this whole exercise with "en" (being worried that Duo would say I skipped translating "en") and it was accepted right along until now. These are the kinds of inconsistencies that are so prevalent and so discouraging. And so unacknowledged. I can't even report it; the option isn't available for this question, not sure why.
"En" -is used as a replacement for an object (to avoid repetition) -And usually used for sentences which speaks of quantity.
For example: Vous avez un pomme? Oui, j'en ai beaucoup. J'en veux un kilo, sil vous plait.
That's based on what i learned in school. Please feel free to correct me if I'm wrong.
The word 'en' can simply mean 'some' but it also refers to a previously discussed object.
For example, if I were to pose the question, "Tu veux du sucre?" (Do you want some sugar) You could reply, "Oui, j'en veux" (Yes, I want some). As you can see the 'en' functions as a reference to the sugar which is our previously discussed object.
I hope this helps!