multiprocessors performance help

In theory you can put as many NIOS processors into an FPGA as the resources will allow. As far as performance, that is going to depend entirely on the design. Your hardware limitations will be fmax and memory interface speed.

The only way you can guestimate is configure a NIOS with approximately the number of peripherals and features that you want, find out how many logic elements/fpga resources that uses and divide that number into the total number of resources on the target FPGA (I would suggest multiplying that by a safety factor of say 50% because of unplanned contingencies and future expansion, probably not more than 70% as then your chip is starting to most likely become pretty congested). That might be a decent first order approximation.