I think the maximum depends on the individual and varies from as little as 3 quarts for some people to as much as 6 quarts for others.
Seems to be a very individual thing
I am highly skeptical of claims of more than 6 quarts.
Clearly not everyone can take that much!
As for reports of people "increasing" capacity.
I'm not saying never, but again take those claims with a grain(if not a substantial bolder) of salt, just because of the number of independent variables.
My own experience has been that, not only is my capacity 4qts but I think that by taking 4qts, I did not increase my overall capacity.
What I HAVE experienced is this:
When I first got my 4qt bag I was in a hurry to try it!
I had to wait until Saturday when I didn't have to go to school, but Dad had to go to work.
I WAS able to fast all day Friday, even at school to prep myself for a Saturday cleanse.
I got to the point of fully cleansed at about 10:30 am Saturday.
I waited for things to settle down and I mixed a very soapy solution for the new bag
I got the whole 4qts in on one valve opening. I can't say there was no pain, I can say that it was not as bad as the worst period I had had up to that time.
I DID retain it 9 minutes, with moderate nausea!
It DID come out clear, so my cleanse had been successful.
It was weeks later during the NEXT time I took a round of enemas that the full effect was felt.
2qts just did NOT feel like it had always felt. It felt like about a quart.
That prompted me to order a 3qt HWB-type bag from the same vendor I had gotten the 4qt from.
Since then I have always needed the 3qts to get the same sensation I previously got from 2qts.
I have used the 4qt bag a number of times since and the pattern is the same.