I gave it a 4. The 24 teams thing seemed a good idea to me at the time, it made qualification more interesting with more teams having something to play for and lead to some interesting teams qualifying, especially good for the Home Nations, I had 3 teams to cheer on. 24 teams also lead to Wales getting to a semi and Iceland getting to the quarters (unless they topped their qualifying group) and those 2 things were the highlight of the tournament so it had a lot of positives.
Unfortunately, it also meant 3rd-placed teams in groups of 4 qualified for the knockout phases, one without winning a single group game, that was poor and lead to matches where teams were unwilling to take much risk, which meant a lot of low-quality matches on display. It also meant a lot more games which were of no interest because they featured two nations I didn't care about like Switzerland and Romania or whatever.