The current setting is 1:
https://github.com/mongodb/mongo/blob/a2af071c5a28f2d92179ac20ac2d163043db19b9/src/mongo/db/s/resharding/resharding_recipient_service.cpp#L207
https://github.com/mongodb/mongo/blob/a2af071c5a28f2d92179ac20ac2d163043db19b9/src/mongo/db/s/resharding/resharding_coordinator_service.cpp#L1044
https://github.com/mongodb/mongo/blob/a2af071c5a28f2d92179ac20ac2d163043db19b9/src/mongo/db/s/resharding/resharding_donor_service.cpp#L229

This means that there's at least 1 thread in standby dedicated just for calling markKilled on the opCtx when the cancellation token is cancelled.