I'm reading a book with this sentence:
Parce que j'en ai une centaine comme ça à la réception.
and according to google it means the same as it is, as with ...j'ai une... instead of ...j'en ai une.... What is the reason for the en usage?
I think this "en" in this case means something like "of these" (I deduce this from "une centaine", which seems to refer to a countable noun in the plural form), and what "these" are depends on the context, i.e., what has been said before.
This would be my attempt at translating the sentence:
Because I have about a hundred of these at the reception.
It is indeed a difficult prounoun and has been from the start for me, and for the same reason you mentioned - we're so often unsure about why it's being used before the verb. After I watched a video I understood it much better, but I still sometimes get a bit confused. I'm actually on active look-out for such sentences, am nothing all of them down and will seek answers. If you haven't already, this video is a must-watch: https://www.youtube.com/watch?v=AJt1vxCmRus