I just started taking insulin, Novolin N (vial contains 100 units/ml, 10 ml)?

There are 100 x 10 = 1000 units per vial.

1000 divided by 12 = 83.3 days ... so the vial should 83 days.

However, as far as I'm aware all vials must be discarded after 42 days even if there is insulin left in it and a new vial must be used.

This paragraph is from the monograph.

Novolin N in use:

Vials

* Keep at room temperature below 77°F (25°C) for up to 6 weeks (42 days).
* Keep vials away from direct heat or light.
* Throw away an opened vial after 6 weeks (42 days) of use, even if there is insulin left in the vial.
* Unopened vials can be used until the expiration date on the Novolin N label, if the medicine has been stored in a refrigerator.

https://www.drugs.com/pro/novolin-n.html