In a 70K cap stadium, I would expect 4-7K Bills fans. Chiefs fans come out big. Hard to drown them out at Arrowhead.
For BBQ: Most of it is pretty good. My favorite will always be Joe's KC (Formerly Oklahoma Joe's), there's several locations and it's fast casual. Q39 and Jack Stack are sit down type, both really good. Several friends also say Slap's is good, but I've never been. There's still a ton I've never been to. You really can't go wrong. Have fun and enjoy the KC