In terms of actual numbers, it's hard to say how many. Because it's a multiple final day, the 64845 obviously doesn't tell you anything about individual support. We would only know if the club told us exactly how many tickets they had sold. Then you'd have to add on some more, for fans who bought their tickets independently.
I think there's a tendency to exaggerate numbers, especially when the vocal support has been good and it's been a great win. You look around you and it feels like the whole ground is cheering with you, when obviously it's not. As a lifelong football fan, everytime I hear someone say "we took 10,000 to so-and-so", I always halve it immediately!
If you really want to try and estimate numbers, take the ground capacity (90,000 in Wembley's case), and try and work out the % of fans. IMHO I'd estimate our support at no more than 5,000, which is superb for a "secondary" cup competition. tbh I don't think it really matters. Personally I judge a fanbase more by it's vocal support, than by sheer numbers. It was a great turnout, we supported our team superbly and everyone had a great day out.
