Designed for 7 so the environmental system can cope. Don’t have the SpaceX suits, but they’re a safety measure not a necessity. Plenty of space where the other seats were so padding could be added in the floor. So it is doable in an emergency.
But so is use of the Starliner itself. The delay is risk mitigation, not that there is a known problem which would endanger re-entry.
If there are doubts the safest option would be for them to remain aboard till the next Dragon crew rotation flight in August. Planned for 4 crew rotation but they could launch with just 2 plus extra suits and recover them with two of the planned returnees, extending the tours of the other 2.
That would reduce the total crew to the previous planned complement.
Starliner could be released before the arrival to do another automated test re-entry, freeing the docking bay.