Seventh-day Adventists church membership comes in at approximately 20 million members. SDA offshoots will no doubt have much smaller member counts.
If you take SDA (and offshoots) out of the picture and just look at other Sabbath observing Protestant denominations, you may find the numbers are quite large. For example, the
True Jesus Church denomination came out of the Pentacostal movement and has 3 million members in 70 countries.
The belief of seventh-day Sabbath observance leads to many other scriptural conclusions. I would say SDA have a lot more in common with other Sabbath observant denominations than not.
Here is some interesting history concerning SDA and the origin of our Sabbath observance.
The pioneers of the Seventh-day Adventist church were from various denominations.
The Thomas Preble
Wikipedia page:
In 1845 the Millerites held the Albany Conference. The
Wikipedia Millerism page:
Joseph Bates became a Millerite preacher in 1839. He read Thomas Preble's tract in 1845 and was convinced of the Sabbath (and so was J. N. Andrews, another Millerite and SDA pioneer). Joseph Bates wrote his own tract in 1846 called
The Seventh Day Sabbath, A Perpetual Sign. James and Ellen White read the Joseph Bates tract that same year. It was because of this tract and subsequent scriptural study that James and Ellen White were convinced of the Sabbath.
Rachel Oakes Preston, a Seventh Day Baptist, influenced Thomas Preble on the Sabbath, according to
Wikipedia:
In 1844, Rachel also convinced
Frederick Wheeler, an ordained minister of the Methodist Episcopal Church, of the Sabbath. He is reputed to be the first ordained Adventist minister to preach in favor of the seventh-day Sabbath.
In 1863 the Seventh-day Adventist church formed.
If I were to give credit to anyone for our Sabbath observance origins it would be Rachel Oakes Preston, a Seventh Day Baptist who was able to convince multiple preachers of the Sabbath.