The number of flights is not a constant, and the number of passengers in a plane is not that big.
If you book a flight, it may push the number of passengers past a critical threshold, and the airline decides not to cancel the flight.
Or, if you don't book the flight, someone else who would have otherwise flown on another flight will be able to book that seat, and the other flight could get canceled.
Or, if some flights are often almost full, your booking may push the demand past a critical threshold, and the airline may choose to schedule another flight. And because there are now more flights, flying that route will be cheaper and more convenient, which may attract new passengers.
If you only fly a few times in your lifetime, you can plausibly say that those planes would have flown anyway. If the number of flights is much greater than the average number of passengers in a plane, your actions have almost certainly increased the number of flights flown.
In some cases. This happened in the EU during the COVID pandemic due to weird regulatory reasons.
But in general: if no one is booked on the plane, they will stop selling that flight pretty quick. Of course in the short term they might fly an empty flight if all passengers no-showed and if they need the plane at the other airport, but their fleet allocation optimizer probably works at most a couple days ahead, so empty flights will soon be culled. Similarly routes with few passengers will get allocated smaller planes in the future, etc, etc.
Airlines often also cancel flights due to lack of demand, sometimes at the last moment, because rescheduling the passengers is more profitable than flying the plane.
If you book a flight, it may push the number of passengers past a critical threshold, and the airline decides not to cancel the flight.
Or, if you don't book the flight, someone else who would have otherwise flown on another flight will be able to book that seat, and the other flight could get canceled.
Or, if some flights are often almost full, your booking may push the demand past a critical threshold, and the airline may choose to schedule another flight. And because there are now more flights, flying that route will be cheaper and more convenient, which may attract new passengers.
If you only fly a few times in your lifetime, you can plausibly say that those planes would have flown anyway. If the number of flights is much greater than the average number of passengers in a plane, your actions have almost certainly increased the number of flights flown.