The seeding will not take place until the regular date after SEC tournament. They are naming 20 potential host sites for Regional and 8 of those will be super regional sites. They are hoping that 8 regional hosts win so they can be the super regional hosts but when applying you agree that even if you are not in the super regional you will still host.
The 20 versus 16 is the fact that a school could not get compliant by the next date or a state could enact a Covid enforcement such as reduce % of capacity that would prevent a school from hosting. That is very relevant since it seems us and Vandy will be hosts, several in TExas and a couple in Florida. So if any state changed their %'s it could reduce the host sites by 2 or more. You have until a given date which has not been posted that I know of to prove your ability to meet #'s of seats and Covid preparedness. I'm guessing they bring in extra seats by Arkansas for this purpose and I'm guessing they are strict on enforcement of NCAA Covid rules for the Arkansas series.