My prospective supervisor already has 8 students and she only contacts me through the assistants so far. I already accepted their offer and now another university is also considering my application, that potential supervisor wrote so many constructive comments on my proposal and wants to interview me. Although I accepted the other one, I'm not sure if I should still go to the other interview. I like how this other supervisor pays attention.
Also should I mention I already accepted another offer?

Generally it's best if they don't have too many, or else you might not get much support. But I would say it depends on:
(1) how good/available your secondary supervisor is. A good secondary can make up for availability issues with the primary
(2) Is the primary supervisor as senior/busy as they are likely to get? It may be that they have time for you now, but if their group gets bigger or they get appointed to an even more senior role (e.g. an institute director) then their availability is likely to drop off.

I'd be worried if a supervisor doesn't contact you directly. It's probably an indication of future supervision.

I would go to the other interview anyway, you've got nothing to lose by doing that. I wouldn't mention you have another offer, just say you've been to another interview, as you don't want to prejudice the outcome in any way.

As a side note, my two supervisors have over 15 students and they still manage to give me excellent supervision. It depends on the person.