We should have around 90k surviving undergraduate alumni. We've been graduating 2000-2200 annually for at least 20-30 years now since the undergrad expanded back in the 80s-90s.
If you assume we've graduated around an average of 1700 undergrads over the last 60 years, you basically get around 100k possible graduates. Of course demographics/mortality come into play which probably brings that to around 90k.
As far as graduate school alumni goes, that number is growing hugely because of all the masters degrees that the university hands out; Northwestern granted 5400 Masters+Doctoral+Professional degrees last year.
Back before the big boom that number was around 3000 graduate degrees conferred annually during the 90s.
Of course that number was also much lower back in the 70s and 80s. Average number over past 55 years (reducing by 5 due to older ages of those alumni compared to undergrads) is probably around 3200 which gets you around 175k graduate alumni; factoring demographics/mortality probably brings that number down to around 160-165k.
So the grand total is probably around 250k surviving alumni at the moment if you're including all alums, and given that the university is handing out around 7,500 degrees annually, the number is likely to grow to around 300k over the next 20-25 years.
But the undergrad alumni number is likely to stay pretty stable, undergrad has been at a stable 2100-2200 new alumni per year for a while and that won't change much; we'll probably hover around 90-100k living alumni of the undergraduate school.