/maxcpucount[:n Specifies the maximum number of concurrent processes to build with. If the switch is not used, the default ...

/maxcpucount[:n]   Specifies the maximum number of concurrent processes to 
                     build with. If the switch is not used, the default
                     value used is 1. If the switch is used without a value
                     MSBuild will use up to the number of processors on the 
                     computer. (Short form: /m[:n])