Six crew members will be sent on two different opportunities to Mars



NASA :
"American manned Mars expedition. After some discussion within NASA, in the Combo Lander mission was found to be too lean. It was again reformulated as a Design Reference-Mission type with two landers, six crew sent on two different opportunities, and ISRU production on the surface, and split cargo/crew landers to provide more redundancy. Even with the mass-saving advanced technology assumptions of the Combo Lander study, this drove the total mass required in Low Earth Orbit back to 600 metric tons, and the number of heavy lift launch vehicle launches to 12."


http://www.astronautix.com/craft/duassion.htm