Officially, the highest first week sales record was held by Adele , whose album "25" sold 3,378,000.
BTS just barely failed to break her record.
There is an unofficial fastest-selling album, namely John Fitzgerald Kennedy - A Memorial Album, released, as you would suppose, right after kennedy's assassination.
It reportedly sold 4 million copies in 6 days (at that time stores were largely closed on sundays and there was no online sales). Costing 99 cents, which was not much money even back then, it included some clips of Kennedy's speeches and report of his assassination with no music tracks.
But now that record is broken by Seventeen, which sold 4,317,000 copies with 2 days remaining in their first week.
I personally am negative about BTS' prospects in 2026 (there would be no 2025 comeback). By then the pop scene will be dominated by others, and other hybe acts might be its biggest obstacle.
Since SVT sold more in the first week than anyone else in the history of humankind, it will never respect BTS again. Never.