The only people who know that are at the BOS as they don't release that information.
My guess, based on my experience of being on one of the judging panels, would be mid- to high 80s but... each course and year varies due to the criteria of the judging (aligning) process and the actual paper, marking criteria and band 6 standards.
It will vary from subject to subject and year to year.
It all depends on the paper each year along with the marking guidelines used by the markers (and the indeterminate figure - how many people the BOS actually want to say have reached the Band 6 level - remembering that they don't want it to appear that standards are slippling).