Clubs have taken that many to Wembley for semi-finals. Some clubs proably did the same in the old, pre-corporate days when Villa Park, Old Trafford, Hillsborough etc hosted semis and tickets were available to anyone.
Was it Cologne who took 20,000 to arsenal last year was it? Seemed a bit odd as surely arsenal only give the standard amount of away tickets so the rest just came for a pointless trip to London I guess?