After Netherlands, I think England should be in line for two referees if that happens above Germany and France (not sure about Spain.)
If Edy's right about 8 from Eng, , Fra, Spa and Ger going then if we presume the Netherlands has 2 as well that's 10 spots gone already, which I think might be a bit much.
If fit, I'm sure Skomina, Orsato, Marciniak, Cakir will go too and I would have said Hategan. So not too many spots left.
Having two refs from so many countries would certainly be a big change from the past - when it happened with Clattenburg/Atkinson in 2016 it felt very much the exception (and not sure it would have happened if Clattenburg had been FA's choice.)