Maybe it's 10 GB for games and 6 GB for system? Sounds overkill for system RAM but who knows?
The digital foundry video explains how much memory is for what.
6x 2gb = 12 gb for "gameing"
1x 4gb = OS ect.
= total 16 GB.
However, dev's can borrow, some of the slower ram (~1,5 GB) if needed.
So that "max" possible ram the Xbox Series X can use (for games) is 13,5 GB.
The "optimal" (max speed) is 12 GB.
Necro-bump this 2020: http://gamrconnect.vgchartz.com/thread.php?id=229249
Bumb in 2021: http://gamrconnect.vgchartz.com/post.php?id=9047071