Basically 'en' is a pronoun that replaces the preposition 'de'. In this case, and most frequently, it is used for quantity. For example, "Do you have any siblings (brothers and sisters)?". Personally, I would respond as such: "Oui, j'en ai 7, c'est trop je sais". So what I'm saying is "Yes, I have 7
(of them)". In English we're lazy and we leave things out of sentences to make them quicker and easier, so we would rarely actually say "I have 7 of them" just "I have 7". But the "of them" is translated as 'en' so, as you can see, you write: "J'en ai 7".
It can also be used in other cases such as with phrasal verbs which are verbs that have a separate meaning when they are followed by a preposition, in this case 'de'. So for example, "parler de" means "to speak about". If I'm worried about my exams (which are fast approaching) and I talk to my friend about them, someone might ask us "Vous parlez encore des examens?", which means "Are you speaking about the exams again?". I could respond "Oui, nous parlons des examens depuis quelque temps" which means "Yes, we've been speaking about the exams for some time".
But it sounds strange to repeat "the exams" and we're lazy so we want to omit words where we can. So in this case it would be much easier to say "Oui, nous
en parlons depuis quelque temps", which would translate as: "Yes, we've been speaking
about them for some time"
Let me know if you need any more clarification
@FrenchUnicorn my pleasure
it's like revision for me