I would like to know if there are reputable sources presenting anything close to an exhaustive listing of the main denominations around the world adhering to seventh-day Sabbatarianism. Similarly, are there recent studies estimating the percentage of Christendom that practices seventh-day Sabbatarianism?
Straight from the churches main website: Seventh-day Adventist World Church Statistics 2018.
And some other data that will get more granular: Browse by Adventist World Divisions.
This is limited to the Seventh-day Adventist numbers, but they are the largest group of Sabbatarians.