To count, she must be a complete airframe and you must have at least walked up the crew entry ladder. I have been lucky enough to have visited:
XH558 - Several times and also on board during Engine Ground Run
XL426 - Vulcan 24hr Scramble
XM594
XM612 - Vulcan 24hr Scramble
XM655 - Vulcan 24hr Scramble
XM573 - Many times in the future!
