A&M doesn't have a trademark on the number 12. They have it on "12th Man."
They're not pressing this in court, but if the Seahawks continue to market stuff based on their fans being the "12th Man" then they're extremely likely to lose in court if it goes that far.
I'm inclined to think A&M has a case. But not a good one.
"12th man" is a concept.
Can I trademark "First born child"?
There are hundreds, perhaps thousands of instances where the idea of a 12th man is relevant.