A billet is a ticket for a single ride. When you use a billet, the chip on your card is given a free transfer (droit de correspondre) that is valid for the next 90 minutes. This means that your ride from A to B will probably only consume one billet. The return trip will require a second billet; even if you start your return trip within 90 minutes of your outbound trip, the rules do not permit you to "double back" without paying again.
Note that at two rides per day, a titre de 5 jours consécutifs ("pass for 5 consecutive days") is a marginally better deal than buying 10 billets ($29 rather than $30.) If you're doing more than two rides per day, the 5-day pass is definitely a better deal. Depending on how much time you have remaining in Quebec City when your 12 billets run out, you might consider buying one of these instead.
Pages I used to figure this out:
Here are the RTC fare schedules in English and in French, which make it clear that "1 passage (billet)" is the same thing as a "single fare (ticket)".
Here's the RTC webpage in English about payment methods, which says
When you use a ticket (single fare) on an OPUS or Occasional card, you automatically get a transfer fare valid for 90 minutes.
In French, the same webpage says
Lors de l'utilisation d'un billet (passage) encodé sur la carte OPUS ou la carte Occasionnelle, un droit de correspondre d'une durée de 90 min est automatiquement accordé.
(This says the same thing as in English.) This last page links to a French page about transfer fares, which does not appear to be available in English; it says
Un droit de correspondre permet de monter gratuitement à bord de tout autobus circulant sur un parcours autre que celui sur lequel il a été émis et autre que ceux utilisés durant sa période de validité.
Billet encodé sur une carte à puce
- Il est automatiquement encodé sur votre carte.
- Valide pour 90 minutes.
which translates as
A transfer permits boarding of any bus running on a route other than the one on which it was granted and any other routes used during its period of validity.
Ticket stored on a smart card
- Transfer is automatically stored on your card.
- Valid for 90 minutes.