Bowker, the private company responsible for issuing ISBNs in the US, released its annual report on the number ISBN's sold last year.
The data doesn't tell us as much as some would think, but it did show that Amazon's Createspace (aka KDP Print) bought the lion's share of ISBNs in 2018. Out of 1,677,781 ISBNs registered in 2018, a total of 1,416,384 were sourced through Createspace. That is a jump of about half a million ISBN registrations from 2017, when Createspace accounted for 929,295 out of 1,192,345 ISBNs registered.
The increase is probably due to Createspace being bundled into KDP last year, and reflects authors taking the opportunity to create print editions for their existing ebooks.
Bowker concluded that the increase meant that "Self-Publishing Grew 40 Percent in 2018", but I would be willing to bet any amount of money you cared to name that they are wrong. (I count print and ebook editions of a title as one, not several.)
You can find the report here.
The data is of course limited to mainly self-published authors in the USA, and is far from complete. For example, there's no mention of Ingram Spark, a POD service that competes with Createspace slash KDP Print. That is probably because Ingram Spark recommends that everyone buy their own ISBN, preferably directly from Bowker.
This report is also missing the many, many, many ebooks that were published without ISBNs (you don't need an ISBN to sell an ebook in the Kindle Store) and it is missing any data from ISBN-issuing agencies in other countries. The rules vary between countries; in Canada, authors can get ISBNs for free, while you basically have to rent them in Germany.
So really, aside from tracking the activities of a couple individual companies, this report is actually about as useful as a wet fart in a snowstorm.
Publishers Lunch may claim that "CreateSpace Continues to Devour Self-Published Print Market", but we don't actually know whether that is true from the data in this report. (It probably is, though.)