If someone on the ISS had a medical emergency that required them to immediately return to earth (such as a severe stroke - something where they could not give them ideal care on the ISS and required urgent care), and it was not during a normal crew rotation (and there was not a launch planned in the next week or two), would all of the residents of the space station be evacuated (particularly, assuming there were only the one lifeboat, and not a spare)? Or would one or more of the astronauts/cosmonauts/etc. remain on board (despite the lifeboat leaving the station)?
All of the crewmembers assigned to the sick crewmember's crew transport vehicle (which as of this writing means the Soyuz) would have to leave as well. Otherwise they would be left without a means of escape in the event of a station emergency.
For a six person crew, with three crewmembers assigned to each Soyuz, three would have to leave.
For the current situation (February 2020), with only three crewmembers aboard, the station would be left un-crewed. A most un-desirable situation, but budget realities and the slips in Commercial Crew vehicle availability have put the program there.
References (sorry, Quora answers, but NASA does not publish ISS Flight Rules, and I used to work with both of the guys that wrote these answers, so I can vouch for them being JSC employees)