[Arch-TK]

The rule is that at all times enough return vessels must be docked to the ISS for everyone on board.

These are usually the same vessels they used to get up to the station.

This has the consequence that if they need to re-dock one of the vessels (for whatever reason) all the astronauts that would normally use that vessel must board it for that menuvre. Just in case it fails to dock again.

And they don't normally have spares.

IIRC, this is a good video on the topic: https://m.youtube.com/watch?v=82YHM12n2JI

[Polizeiposaune]

There is spare capacity on the Crew Dragon for an extra astronaut or maybe two on return. They'd rather not have to use it but NASA took steps to enable it when Soyuz MS-22 suffered a coolant leak in 2022 and had to be returned empty.

https://en.wikipedia.org/wiki/Soyuz_MS-22