The European Cup started in 1955 and, in 1992, was renamed the UEFA Champions League. What's the record attendance for an EC or CL match during those 70 years?

Per the wikipedia page a whopping 135,805 in a Celtic v. Leeds United match in 1969-70 during the semi-final. I can only assume that's during the grandstand era because that's way above the seated capacity. The Hampden Park wiki page gives the attendance for that match at 136,505, but right in that 136,000 range seems to be correct. With that many people, taking accurate counts might have been difficult.

Also, in case you were meant the record LOW attendance, 0 - for the final during 2020 the COVID-19 pandemic between Bayern Munich and Paris St. Germain.

https://en.wikipedia.org/wiki/European_Cup_and_UEFA_Champions_League_records_and_statistics

https://en.wikipedia.org/wiki/Hampden_Park

Celtic v Leeds in the 1969-70 at Hampden Park, official attendance was 136, 505, although it is generally thought that many more climbed the walls and didn't pay their way in.

https://en.wikipedia.org/wiki/Scottish_football_attendance_records#European_football