It's an interesting point about the expanded format of Euro 2020 in terms of being across 12 host cities. I think that might increase the number of FOs and VARs (if VAR isn't done from one central location.)
I still think 18 referees is probably about right though - that's 2 games each in the group stage. There shouldn't be any issue with an official refereeing in Baku on the opening Saturday and Bilbao a week later for instance.