I was doing some research regarding the Soyuz as I'm currently planning a summer project in which I would build a close replica of a Soyuz when the question popped into my mind, "What would happen if there was a freak accident in which that the ISS needed to be evacuated and there was ONLY one space craft available?" I'll leave y'all's response on the matter to either be regarding the Crew Dragon or the Soyuz.
By design that will never happen. There are always enough return seats for the crew.
This is exactly why
the whole crew of one of the visiting vehicles gets in it whenever it undocks, even when it is only being moved from one docking port of the station to another
the crews retreat to their vehicles in fire / leak / toxic atmosphere emergencies
the crews retreat to their vehicles if a collision risk is discovered too late to execute a safe distancing maneuver
What would happen if there was a freak accident in which that the ISS needed to be evacuated and there was ONLY one space craft available?
Carrying that to an even greater extreme, what would happen if there was a freak accident in which the ISS needed to be evacuated and no space craft was available? For example, suppose two human-qualified vehicles are docked with the ISS. Suppose an unseen 10 cm diameter asteroid hits one of those escape vehicles at 30 km/s. It would rip right through that vehicle. Suppose it ripped open a propellant tank. Kaboom! Suppose some of the resulting debris opened holes in the ISS, and other chunks opened holes in the other escape vehicle.
The answer is simple: The astronauts and cosmonauts on the ISS will die. Being an astronaut, cosmonaut, or taikonaut is not exactly the safest career choice. One of the standard questions potential astronauts are asked is whether they are willing to die given that over 10% of astronauts have died on the job. Potential astronauts have to be willing to accept that possible outcome. They also have to be not willing to desire that possible outcome.
The ISS is intended to be two fault tolerant regarding loss of life at the three sigma level with respect to reasonably plausible failures. The ISS is not even one fault tolerant with respect to some outrageously implausible root causes such as my hypothetical 10 cm asteroid. It's also noteworthy that the three sigma level is not that high of a bar; modern Earth-bound manufacturing is moving toward making the devices they make safe and reliable at the six sigma level.
Bottom line: There are semi-plausible scenarios where astronauts or cosmonauts will die. This is a risk that astronauts and cosmonauts sign on for, but it also is a risk that they work very aggressively to counteract.
There WAS a different concept for returning folks from orbit in emergencies that was considered, called MOOSE, but it never got out of the planning stages.