I was at the last game as well and I would guess it was 30-40% Seahawks fans. When we moved here to Minneapolis, a couple years back I couldn't believe all the Seahawks fans. There is a strong bandwagon component here due to the Vikings being so mediocre for so long.
How many? On one of the Viking forums, someone posted a thread begging people to stop selling their tickets - seeing quite clearly who was buying them up. 12s will be well represented and at times it will sound like a home game.