Questions About EB1B

sorry, I am newbie here. Just considering to apply for it.

For EB1-B, how many references do we usually need?
And are they needed to be from different institutions?
and pretty strong?
If no strong recommendation letter writers, should we just ignore EB1B and just go EB2?

And in which stage we need to reference letters?

BTW, what is PD and RFE ppl use on this website?

Thanks a lot.