Microsoft's understanding of average Xbox gamer habits, based on anonymized data taken from years of console use, suggests that an average user has an inventory of roughly three to four games they frequently return to. As a result, at a bare minimum, Xbox Series consoles should retain three to four "next-gen" game save states for the sake of Xbox Quick Resume—though, thanks to my back-compat limits, I've been unable to test this claim as of press time.
Instead, I've pushed my tester Xbox Series X console with a mix of XB1, X360, and OGX content to test how many save states I can cram into the console's internal memory before the feature stops working. My average result has been 12 games. Meaning, I can manually load 12 games, then go back in order to reload each of the dozen, and they'll each have a brief "Quick Resume" tag appear on the screen before reappearing in fully playable, middle-of-the-game action within five to 12 seconds.