Each crew that will go to Mars consists of four individuals. Four is the smallest possible group size because of the different skills that need to be present in the first team leaving for Mars. Working with a larger group would increase the cost and complexity of getting to Mars.
The astronauts will have trained for many years together before even setting foot in the transport vehicle. Groups will be composed based on personalities, abilities, and chemistry, in order for each group to work together in the best way possible. If one astronaut in a group changes his or her mind about going to Mars, or obtains an illness which prevents him or her from participating in the training program, or deemed not to be suitable for the mission, the whole group will leave the program or will start again from the start of the program with a new team member.
For this reason, there will always be new selections and several groups in training for future missions. This will make certain there are always qualified and integrated teams on stand-by, preparing for their time to travel to Mars.