Distilling what we know, we know for sure that only 13+ members have had the opportunity to do Fan Update so far. All other members have been able to continue to buy without doing Fan Update (as evidenced by RebeccaLFC's post at 18:57:23 above).
The number on 13+ is either 5,000-ish or over 8,750. (The latter being necessary, for 80% of it being greater than the 7,000 figure quoted by the Club as having done Fan Update last month.)
Maybe the 7,000 figure in the 3 December entry here
https://www.liverpoolfc.com/tickets/fan-update-blog is a typo for 4,000. (4 and 7 are next to each other on the numeric keypad.)
That would make sense because most Fan Updates were probably done by then. The final number could have been just over 4,000 out of 5,000-ish total. The 1,000 shortfall gives the 20% figure the Club are quoting today.
Of those 1000 (who all bought 13+ tickets last season), just 100 bought in the November sale and 900 didn't bother for what is a very high-demand game.
Those of us on 13+ were competing with 900 fewer people in the November 13+ sale. I fear the Club will use this as justification for reducing the threshold from 13 to 7-ish games to get the number for the guaranteed sale back up to 5000.
It would be good if Graham can get the Club to release the actual numbers:
1) On 13+
2) On 13+ and completed Fan Update