The BeMicro uses an EP3C16F256C8N. So the correct question is how many NIOS II processors can fit inside that device.
Here is the information for the Cyclone III:
http://www.altera.com/products/devices/cyclone3/overview/cy3-overview.html The device has ~15,408 logic elements
Resource utilization for the NIOS II is best estimated using the NIOS II megawizard inside SoPC builder. However, this documents gives some info:
http://www.altera.com/literature/ds/ds_nios2_perf.pdf So assume you are using the NIOS II/f implementation (~1800 logic elements). Theoretically you could fit 8 cores inside your Cyclone III. Now in reality there is a ton of other logic (Avalon interconnect fabric, SRAM cores, timers, and whatever else you've got in your design). What I would do is look at your current project and see what the resource utilization is and how much space you have left in the device. Then assume something like ~2000 LEs to add another core.
Jake