Which N-form applies depends entirely on why the tenancy is ending — using N12 language for what's actually an N4 non-payment situation (or vice versa) is one of the more common reasons a landlord's application gets kicked back at the LTB.
The forms themselves are static PDFs from the LTB — the operational risk isn't finding them, it's tracking which notice was served on which tenancy, on what date, and whether the required notice period has actually elapsed before the next step. That's a per-tenancy audit trail problem, which is exactly the kind of thing that gets lost between a spreadsheet and an inbox once a portfolio passes a few dozen units.