It is 5% in the Champions League. So it probably are about 3500 tickets available for guest fans in the Allianz Arena.
Informations about the arena in english:
https://allianz-arena.com/en (how to pay inside the arena, what items aren't allowed, seat plan, parking information, how to get to the arena, luggage storage etc.)
PDF Guest Info:
https://allianz-arena.com/binaries/...stinfo_fan_oktober2021_en.pdf?v=1678113216889
Parking info regarding the number plate recognition and payment:
https://allianz-arena.com/binaries/...nerkennung-gaesteparkhaus.pdf?v=1575883343486
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Düsseldorf is about 500/600 km away from Munich. If you look for different airports to Munich you might look into Salzburg, Nuremburg, Memmingen (Allgäu Airport - but they only have a direct line to Stansted) maybe even Stuttgart or Innsbruck. From there you might be able to get cheap tickets via Bahn (Bayernticket - only regional trains) or cheap busses (Flixbus).
For tips about food/beergardens/short trips etc. around Munich you might like to look into the videos that an English and American couple makes that live in Munich.
https://www.youtube.com/@NearFromHome/videos
And - what I told in another thread: On Friday after the match the Springfest starts on the Theresienwiese. It is not as touristy as Oktoberfest, smaller and lesser known in the world - but it is fun, too.