25,000 at most, rattling round a 3/4 empty stadium, with the fans travelling 200 miles return, think of the cost to them, plus they won't get that much financial reward due to the staging costs that come with Wembley ...to say nothing of the CO2 etc..
Once the play-off contenders are known, it would be much better to put 2/3 Premier League grounds on standby, roughly equi-distant between potential finalists. Once the two finalists are known, play it there ....
Salford City v AFC Fylde at a sold out Turf Moor would provide a much better atmosphere ... according to the AA, it's 39 minutes drive from Salford's ground, and 42 minutes from AFC Fylde's.

