Grad students: Help me understand the credit hour system. Thanks!

Hi, recently I was talking to my grad advisor and she mentioned that the recommended credit hour per semester is 9-12, beyond that it's super difficult to manage. But I spoke to my seniors who have graduated already or are on the verge of it and they said they had taken 15 or some even said 18 credits per semester. I was shocked to hear this since it goes way beyond the recommended amount. But for people who want to get the dual degree (MS / MBA which is a total of 63 credits in 4 semesters) the minimum credits to get done is 15.75 per semester so it's kind of a compulsion to do at least 15 credits in some and 17-18 in some semsters.

I want to understand how does the credit hour system work? What is the real time equivalent of 3 credit hours? (I come from a different country where credit hours is not really explained or given much importance to so I don't exactly know how the US credit hours system work).

How do some people manage to get done with so many credits in one semester? And is it possible for an average student to do 15 credits in one semester?

Thank you for helping! :)