I suspect that was Duxford, September 2000.
You may be right but, from memory of my OFMC days, I think there were 20 Spitfires in the formation on both days in September 2000.
I certainly remember the excitement that it would be the biggest post war 'wing.'
The plan was to have 21 but one didn't arrive until the Sunday.
The 21 total wasn't achieved on Sunday because one of the BBMF pilots was sick and unable to fly so it was 20 again.
I don't think the 20 record has been broken.
FL