Yes your bang on, I think what the person who told me means is that they won't sell their entire allocation.
Hmmm... I get you. They are definitely wrong though and they'll sell the allocation easily. Their average home attendance is about 24k this season so an extra 4k is absolutely nothing for an FA Cup Final.
Since the new Wembley opened we've had Millwall take almost 50,000 fans for a JPT final and Bristol Rovers take 40,000 for a league 2 final. It's bonkers really when you consider their average attendances.