All trains, not just Acelas, are at 90-100% full every day, at least the WAS-NYP corridor. It will be a real bummer if they can't get the staff to bring back more trains before 2024.

I only use upgrade coupons on trains that are 80% full (you sometimes see that 48 hours before), never on Fridays or Sundays, and sometimes I'll purchase a FC ticket and immediately get a refund so I can see the seating situation.