The most frequently played match in European Cups is Bayern Munich Vs Real Madrid they have met 24 times up to now.
uefa.com article states:
Bayern and Real Madrid have met 22 times previously in the European Cup, ...
foxsports.com article published on Apr 12, 2017 states:
No fixture has been played more often in the Champions League or European Cup than this one. Wednesday’s game will be the 23rd meeting, and the 21st in the knockout stage.
bundesliga.com article states:
Bayern Munich and Real Madrid meet for a record 23rd time in European competition ....
americascardroom.eu article states:
This match will be the 23rd time both sides face each other, making it the most played match-up in the UEFA.
Other: Bayern Munich vs Real Madrid is most played game in Champions League history
Results of past matches: Source Other