Right now, orbiting at 400 km above Earth's surface, we have a permanent crew of six astronauts at the International Space Station, a first in the history of space exploration.
Both the Russian MIR and United States' Skylab had a permanent crew of three. The International Space Station also had three astronauts until last Friday, when ESA astronaut Frank De Winne, Russian cosmonaut Roman Romanenko and Canadian Space Agency astronaut Robert Thirsk arrived onboard the Soyuz TMA-15. Together with the other three existing members, they have become the Expedition 20 mission crew, the first 6-member resident crew ever in a orbital outpost. [Hyperbola]