Maybe, but the constraint is usually the feeder to the subpanel, not the empty slots in the garage can. A 60-amp or 70-amp garage feeder that already serves lights, a freezer, and a door opener may not have room for a 32-amp or 40-amp continuous charger. Even if the feeder is large enough, the main service still has to pass a load calculation that includes the new EVSE. You can have space in the subpanel and still be out of capacity in two places.
A simple educational sketch, not a permit form: a garage subpanel on a 60 A, 240 V feeder already drawing a measured or calculated 20 A has 40 A of apparent room, but a 32 A continuous charger typically needs a 40 A circuit and, in a measured-demand style check of the feeder, is added at face value (and some electricians will want still more conservatism). That 60 A feeder can go from “fine for garage receptacles” to “too small” in one step. A 100 A garage feeder is a different starting point.
Have the electrician check three ratings in order: the EV branch circuit, the subpanel bus and its feeder overcurrent device, and the serving service. If the feeder is the bottleneck, options include a feeder upgrade, landing the charger in the main panel instead, a lower charger setting, or listed load management. Use the calculator for the service-level arithmetic, and do not treat garage breaker spaces as the answer.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.