Its not essential that a power plant uses only two rectifiers, it depends on the load there at the site and the capacity of the rectifier or SMPS. Assume the load at the site is 120 Amps and the SMPS capacity is 2000W or the current capacity of the recifier is 50 Amps. In that case 3 or more rectifiers is preffered. Rectifiers is used to stability the load in each phase.