The stadium 'Artemio Franchi' had a capacity of approx. 47,000 with typical attendance of between 35-40,000 but would only sell out for big league and Euro matches.
This season, due to whole stadium renovation, we're at roughly half capacity - around 22-25,000. The Curva Fiesole (ultras end of around 10,000) is closed and been moved to the opposite end, next to away fans, so this complicates segregation arrangements and capacity, depending on away team and size of support.
As a one team city of around 360,000 people, the fan-base is 7th or 8th largest in Italy I think, and we have a lot of worldwide fan groups now too.
I presume for Selhurst, the away allocation will be around 1500 ish? We have a lot of demand judging by comments I've seen and will no doubt sell out whatever we're given. Just hoping to get tickets!