If your OS is able to handle more than one CPU (NT4, W2000, Linux, BeOS (not the personnal edition)... and NOT W9x( including ME)), then the RC5 client will detect it and automaticaly launch X crunchers.
Running more than one instance of the client on a single CPU machine is useless. The rate is going to be divided by X.
In this case, you'd better have to let the client decide.

You could force it if it don't detect the good number of CPU.