Seventh‐day Adventists are definitely the most prolific Christian keepers of a Saturday sabbath, but they are not
totally alone. The next most prominent groups I can think of are Seventh Day Baptists and Messianic Jews. SDAs are in the tens of millions worldwide, Messianics in perhaps the hundreds of thousands, and SDBs under one hundred thousand.
Although they’re fairly distinct traditions, in my experience each places great importance on religious liberty and separation of church and state, because historically all three groups have faced governmental persecution from blue laws. SDAs go so far as to believe Revelation’s mark of the Beast refers to a blue law to come in the future.
Messianics also have been known to face religious persecution in Israel. To the extent Christianity is viewed with mistrust by Judaism due to Christendom’s history of persecuting Jews, Messianic Judaism is further seen as an attempt by Christians to co‐opt or corrupt Jewish practices.
(Another fun fact: Seventh‐day Adventists hyphenate their name, but Seventh Day Baptists don’t.)