Once.

Right now??? None. lol
I usually 3 days minimum. And that's purely based on my shift work. I think if you train less than that, you can't complain that you feel lost.
I don't roll every time because my back is fucked, but I ALWAYS go to class. So many blue belts and up skip class and just come for rolling. I'm the opposite. :)

3 classes a week to make progress.
2 classes a week to hold steady.
1 class a week to keep in touch.
That's what I've lived by. Over the years I've gone in and out of each of these a number of times. The most important thing is just don't quit!

Anyone can say they train. Easy indicator is to simply step on the mat with them. Time on the mat shows.

I think that there is a big difference between training and just turning up...
I would say if you are present and activily participating in the lesson and drilling what is taught to you at least once per week then you train BJJ. I have a friend whom due to family commitments can only be at one training per week but he makes the most of that one session and has progressed doing so. I also have had students who turn up every session available but waste their time and mine and try and smash their way through rolling for a win...
I think the better question here is:
How invested in your training do you need to be before you can say "I train bjj".
My answer to that is: once