I'd be shocked if it didn't pass 20 million.
Forza Horizon 5, which you would think is a much smaller game, hit 18 million players in less than 3 months. Between GamePass and Bethesda's big presence on PC, Starfield should hit 20 million players by year's end no problem. I also expect its sales to players ratio to be notably higher than FH5, and MUCH higher than Halo Infinite.