The SCA publishes historical membership numbers on its website, but sadly those are posted as individual PDFs by year, which makes it hard to view the long-term trends.
(There are also a few unfortunate gaps, including all of 2021, and a few places where the data seems anomalous, like Northshield in April 2022, but there’s not much I can do about that.)
In order to make such long-term analysis easier, I’ve copied the numbers from 2013 on into a single spreadsheet. I’ve attempted to adjust for variations in formatting; please let me know if you spot any transcription errors!
Here’s a high-level chart by kingdom (click to embiggen):
And here are the total numbers across all kingdoms, but not including members of the overseas affiliated organizations: