The maximum number of crew who have inhabited the ISS during a crew rotation is 13.
That total occurred during Space Shuttle Endeavour’s STS-127 mission in July 2009. The shuttle carried seven astronauts, while six long-duration Expedition 20 crew members were already aboard, creating a temporary population of thirteen. The overlap lasted roughly eleven days while Endeavour was docked.
Thirteen was an unusual handover-era peak, not the station’s standard working crew. Before the shuttle era ended, visiting shuttle crews could temporarily double the population. After commercial crew spacecraft began flying, the ISS standard expanded to seven permanent crew members, matching its original design capacity more closely.
A common mix-up is confusing the maximum temporary population with the normal expedition size. The ISS has also housed smaller crews, including two-person caretaker crews after the Columbia disaster, when shuttle flights were suspended and supplies were limited.