I think aside from Sora and Riku, there would probably only be 3-5 Keybladers in total, maybe 7 if we can figure a plot reason to have that many. I think we should severely limit, though. Make Keyblades and their wielders mean something. Too many KH rps that I have been in either had bullshit reasons for Keybladers or there were so many that the original purpose of Keybladers meant nothing.

As for your second question, absolutely. Our mains will definitely be Keyblade kids.