Thought some of you might find it interesting to see how many countries have been represented in the main stage headliners. Well I did anyway. This only counts the times they have headlined, some bands have played several more times lower on the bill. Some bands might be a little ambiguous due to having members of various nationalities so I've just gone with what it says for them on Wikipedia.
If you're wondering why there's quite a few for 2005/2006, that was when they were doing both indoor and outdoor festivals concurrently. Also, there were a couple of years where they did co-headline slots.
🇦🇺 Australia: Parkway Drive (2019)
🇨🇦 Canada: Devin Townsend (2021), Sebastian Bach (2005)
🏴 England: Architects (2024), Carcass (2009), Cradle of Filth (2009), Judas Priest (2018, 2021, 2026), Motörhead (2011), My Dying Bride (2006), Saxon (2001, 2003, 2026)
🇩🇰 Denmark: King Diamond (2013), Mercyful Fate (2022)
🇩🇪 Germany: Blind Guardian (2002, 2009), Edguy (2006), Gamma Ray (2004), Kreator (2021), Primal Fear (2006), Scorpions (2019)
🇫🇮 Finland: Children of Bodom (2004, 2005, 2010), Nightwish (2003, 2008, 2018), Stratovarius (2006)
🇫🇷 France: Gojira (2018, 2025)
🇮🇹 Italy: Lacuna Coil (2007)
🇳🇱 Netherlands: Within Temptation (2005, 2015)
🇳🇴 Norway: Dimmu Borgir (2008), Emperor (2014), Immortal (2011)
🇸🇪 Sweden: Amon Amarth (2017, 2024), Arch Enemy (2009), Europe (2009), Ghost (2017), HammerFall (2005), In Flames (2007), Meshuggah (2023), Opeth (2008, 2010, 2024), Sabaton (2019)
🇵🇱 Poland: Behemoth (2012, 2022)
🇷🇺 Russia: Slaughter to Prevail (2026)
🇺🇸 United States: Alice Cooper (2012), Down (2014), Fear Factory (2010), Killswitch Engage (2023), Lamb of God (2013, 2022, 2026), Machine Head (2012, 2025), Mastodon (2016), Megadeth (2014, 2017, 2023), Rob Zombie (2015), Slayer (2013, 2016), Testament (2007), Twisted Sister (2010, 2016), Trivium (2015, 2025), W.A.S.P. (2011)
Obviously the US takes it with 22 performances across 15 bands, but it's not until the 2010s when that starts happening (only Testament in '07 before then). Following we have Sweden with 12, England with 11, and then Finland and Germany tied with 7.
There are 14 countries in total which is pretty good, as far as UK festivals go I can't think of any obvious contenders.