Last time the station briefly hosted 9 astronauts before today's (September 4 2015) One-Week Handover, was on November 2013 (for 3 days, Nov. 7 - Nov. 10) before the 2014 Sochi Winter Games when the Olympic Torch was in space for the first time in history. Before that, without involving publicity stunts, the final flight of the Space Shuttle, STS-135 (Atlantis) carried a crew of four, which joined on 10 July 2011 the Expedition 28 crew of 6 on station for a total of 10.
During final assembly of the station and before Space Shuttle fleet retirement in 2011, the station often hosted 9 or more crew at the same time. For example, the final mission of the Space Shuttle Endeavour, flight STS-134, delivered a crew of 6 joining on 18 May 2011 first the complete Expedition 27 crew of six for the total of 12 crew members on the station, before the Soyuz TMA-20 departed on May 23 with thee Expedition 27/28 crew members, for a total on station of 9.
That was dubbed in the media as space traffic jam and it had some interesting side effects, like not all of the crew on the station being able to also sleep on it. But it actually wasn't the first. I think that the record was 13 with STS-131 (Discovery) crew of 7 joining another crew of 6 on 7 April 2010 for a total of 13 (as shown in this picture):
STS-131 and Expedition 23 crew members gather for a group portrait in the Kibo laboratory of the International Space Station while space shuttle Discovery remains docked with the station. Image credit: NASA