Following the game's full release after a five-day early access period, Microsoft stated that one million players were exploring Starfield's planets at the same time on September 6.

The details:

Bethesda's latest RPG, Starfield, surpassed one million concurrent players on launch day across all platforms, including PC, Xbox Series X/S, and Game Pass, according to Xbox boss Phil Spencer. “Starfield exceeded 1 million concurrent players across all platforms today,” Spencer said. “Thanks to all the players who helped us reach this great milestone and congrats to the Bethesda Game Studios.”
Previously, the game had over 230,000 concurrent players on Steam during the first few hours of its early access period on September 1, with a peak count of over 240,000. After the game's general release on Wednesday, 269,177 users were playing it on Steam, accounting for a sizable portion of the one million players.
So far, Starfield has been positively received, with an average score of 87 out of 100 from 118 critic reviews, according to OpenCritic. Many have praised Bethesda for remaining true to its traditional sandbox RPG style while improving combat, quests, and exploration over previous releases.

More Starfield news:

Bethesda's Pete Hines has stated that the company "embraces the chaos" caused by bugs in its games and that the company has always prioritized player freedom and fun over fixing minor issues. Starfield has so far been regarded as one of Bethesda's most stable releases to date, although even Hines himself helped test the game before launch.
Respected Skyrim and Fallout 4 modder Elianora revealed that she had been secretly hired to work on the cluttered environments in Starfield, which have been well-received by fans. She pretended to be surprised by many of the game's showcases, including examples of her own work, to avoid revealing that she was working on the title.

Microsoft claims one million concurrent players played Starfield on launch day