We need the numbers left which we have - 13,500 according to the figures above.
We need the rate at which they are selling. We really need someone who has been following the sales day by day to say how it's going because I think your logic that the sales rate would decline as sales progress is sound. I don't know the daily sales but we can roughly estimate it...
The original allocation was 33,534 and sales started on the 5th Feb so we have had 6 days of sales and if we assume that the entire remaining allocation is online then we can say that 20,000 have sold over 6 days which makes it 3,300 sales per day. But I suspect daily sales per day are now a lot lower. If we assume that a 1,000 sell per day on Tuesday, Wednesday and Thursday then there'll be about 10k going to Cityzens on Thursday afternoon but that's based on a very shaky understanding of how they are selling now. As you said, the rate will not be constant, and we need a recent daily sales figure to get a good idea.
if the current count is 13,500, let's see what is it as this time tomorrow, and then that will give us a good idea as to how fast they are selling, and what will remain on Thursday afternoon.
if someone has been counting the seats sell, then they've probably got a good feel for how many are selling per day and then we will have a good idea. It's 99% certain there will be thousands going to Cityzens but to be more precise we need more data! The Millwalllveteran could be on the money.