Its more than 20k, more like 30k I would suggest as the 3rd tiers only hold about 18k, and EL3 has already sold quite a few and only went on sale today. I'm no seat counter, so someone else can probably be more accurate.
I was hanging on for the fixtures to come out, as my travel means an overnight stay for midweek games, so its pretty much 2 days out of my life for a midweek home game. I do want to go, but there could be other home league games midweek + CL, and I can only do so many midweek games.
Its certainly selling well though, considering its 4 months away.