12,000 is its capacity. That would be like point out BYU football only drew 6,000 fans playing a game out South Field.
Tickets at the temporary field are crazy expensive by NU standards. Northwestern is always going have demand for at least 20-30K visiting fans per conference home game with all the B1G alums living in Chicago. If NU ever becomes decent again, that place will be sold out since NU will only need to draw 10-15K home fans.