The 20million number comes from the Mixpanel data for Xs Max. The article states that the Max represents 2.81% of the installed user base. I think trombone used 715,000,000 as the estimated user base (I've seen that number from an analytical firm back in early 2017). 2.81% of 715million is roughly 20million Max units.
I use the Mixpanel data differently and come up with the following.
I'm using 675,000,000 as the estimated installed user base. Source -
https://www.businessinsider.com/apple-installed-base-iphone-8-stopped-growing-2017-7
Xs - 2.26% of installed base
Max - 2.81%
Xr - 1.66%
This would imply unit sales of ...
Xs - 15 million
Max - 19 million
Xr - 11 million
Total - 45 million, through Dec 10th.
There's approximately 90 days in a quarter. Dec 10th is approximately day 70. So I factor in 1.28 for the days remaining in the year. The totals are now.
Xs - 19 million
Max - 24 million
Xr - 14 million
Total - 57 million
Here's where we get dicey. What percentage of new phones sold are the older ones?
I can use 80-20 or 70-30.
80-20 mix - 71.2 million iPhones
70-30 mix - 81.4 million iPhones
The last 2 Christmas quarters, Apple sold around 79 million both times. At an 80-20 mix, Apple disappoints dramatically. At the 70-30 mix, it's an all-time record.
Here's where more insight/data is required. Knowing the new sales of the older models would be very helpful in determining the "mix". The methodology I used locks in the unit sales of the new models, so the lower the mix, the more total unit sales.
Anyway, this is what I could put together in ten minutes. If I had more time, I could do a little more googling to see if there are indicators of 7/8 sales.
EDIT - usually around this time of year, CIRP releases a chart showing the breakdown of new iPhone sales by model (for US). Typically for the Dec quarter, the breakdown of the new models vs. the old models is between 60/40 to 70/30. I think the last time the mix was in the 80/20 range was in 2014 with the 6/6+.
Opinion - I think Apple will be fine with unit sales.