Previous leaks have suggested that the "Galaxy S21 Ultra" will have a battery of the same size as its 2020 counterpart: 5000mAh. Now, the regulatory body 3C has reportedly produced some more evidence of this possibility. The new registration made by Samsung is for a model number that may match its upcoming flagship smartphone.
https://www.notebookcheck.net/A-new-leak-may-confirm-the-Galaxy-S20-Ultra-s-successor-will-not-get-a-battery-upgrade.496355.0.html