For your typical A320, B737 operation, the minimum number of Captains and First Officers per aircraft is 5 each, so that's 10 pilots
1 CPT / 1 FO flying it right now
1 CPT / 1 FO just having flown it
1 CPT / 1 FO who will fly it next
1 CPT / 1 FO who are on standby
1 CPT / 1 FO on annual leave (and therefore you might be able to offer them overtime)
As you guessed, the reality is something else. Many airlines are trying their luck with 3 or 4 CPTs / FOs per aircraft. Worse, some have outsourced 20% of their operation to wet lease partners who don't even have 3 or 4 CPTs / FOs per aircraft. I left such a company who cancelled 3 flights on behalf of their "customer" airline in a week because I called sick. Some airlines (1) with economies of scale on their side can absorb compensation claims and would rather pay that than hire more crews because that's a longer term burden in case they need to downsize in a hurry. Smaller airlines (2) can't hire afford to hire more crews given the price most people pay these days for a flight. They can't raise those prices because of (1). The economics of the airline industry are broken.