Its not necessary that a power plant uses only two

rectifiers, it depends on the load present at the site and

the capacity of the rectifier or SMPS. Suppose 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

balance the load in each phase.