Primer is estimated with the same arithmetic as paint but a different coverage number, and using the paint figure is why people come up short. Primer is formulated to grip and to seal, not to spread, and on bare drywall it can drop under 200 square feet per gallon.
When primer is actually required
Bare drywall always. New drywall and joint compound absorb at completely different rates, and paint applied straight onto them dries to two visible sheens — the joints show as bands under any side light. This is called flashing and it cannot be fixed by another finish coat.
Bare wood always, to stop the grain raising and to seal knots. Knots specifically need a shellac-based or stain-blocking primer; a general purpose primer will not stop a knot bleeding through.
Water stains, smoke, crayon and marker need a stain-blocking primer. Latex primer over a water stain will let it through within weeks.
Glossy surfaces need a bonding primer or they need sanding, or the finish coat will peel in sheets later.
When you can skip it
Repainting a sound, previously painted wall in a similar colour does not need primer. A good quality paint on a clean, matte, intact surface is fine on its own.
That is the honest answer, and it saves people real money. Primer is not a virtue in itself — it solves specific problems, and where those problems are absent it is an extra day for nothing.
Tinting primer is the cheap trick
Most paint counters will tint primer to a grey scale or toward your finish colour at little or no cost.
The reason it matters is that deep colours have poor hiding power. A saturated red over white primer can need three finish coats. The same red over a primer tinted to a mid grey usually needs two.
A finish gallon costs considerably more than a primer gallon, so trading a tint for a coat is straightforwardly good arithmetic.
One coat, in most cases
Primer is a single coat in almost every situation. Its job is to create a uniform, sealed, grippy surface — not to hide.
The exception is a severe colour change or a heavily patched wall, where a second primer coat is cheaper than a third finish coat.
The calculator defaults to one coat for this reason. Change it if you are covering something difficult.
Do not thin it to stretch it
Thinning primer to make a gallon go further is a false economy that shows up as a weak bond. The primer film ends up too thin to do what it is there for, and the finish coat carries the consequences.
If you are short, buy another gallon. Coming up half a gallon short on a job is annoying; having the whole wall peel in eighteen months is worse.