It sounds like Starfield is off to a very strong start, with Xbox Phil Spencer sharing an impressive figure of concurrent players.
Starfield has obviously been in the hands of some players for almost a week now, but the majority of us were only able to start playing yesterday. Unsurprisingly, that also seems to be where the game has hit its highest player count, with Spencer posting on his personal Twitter account that "Starfield exceeded 1 million concurrent players across all platforms," on launch day. "Thanks to all the players who helped us reach this great milestone and congrats to Bethesda Studios."
It's unclear how this figure is distributed across Xbox and PC, as the only concrete figure outside of Spencer's we have is from SteamDB, which shows that Starfield hit a peak of 269,177 players yesterday. Starfield is also available through Microsoft's PC store, but we don't have any numbers there. It is likely that a majority of players are playing on Xbox, though, especially considering the title is available through Game Pass.