Yes, I think 10K is probably a good start. 25K? Probably not. Yet. IMO of course, and I haven't seen any study on which to base my opinion.
My point is, just because one stadium has X capacity and they allow X number/percentage of people in, you can't simply say "The Bills should let the same number in, based only on capacity." It's more complicated than that. Somewhere between 9-12K, or thereabouts, just seems like a good place to start. Again, without having real data to look at.
And, if I'm not mistaken, the fans are required to ALWAYS wear a mask, unless actively eating/drinking.