This is directly from the NU website and it is essentially what I remember...
"A student entering as a transfer student in a four-year degree program must be registered at the University for at least 6 quarters and earn credit for courses worth at least 21 units at the University."
"Between 42 and 50 units are required for graduation, depending on the Northwestern undergraduate school."
side note - (It is possible the requirement of 21 units was lowered)
"The official evaluation of your credits will be made by Northwestern's Registrar's Office. Once a transfer applicant has been accepted, the Office of Undergraduate Admission will initiate the process of credit evaluation."
admissions.northwestern.edu