It is possible to add thousands of RPCs. The number is completely dependent on the amount of "Memory RAM & Video Memory" available on your Machine at render time.
Once memory is depleted you will receive an "out of memory message" and the RPCs will be unloaded from the scene. This is not an issue for most 64-bit users.
ArchVision Support has encountered projects with 500+ RPCs without any issue, and some other projects with memory issues with just a handful of RPCs and complex rendering calculations.
NOTE: HI-Res and 3D+ RPCs, can generate a higher memory usage and affect the Software performance when using "Realistic Mode" on REVIT.