*Usually 5-7 refs.
And are they needed to be from different institutions?
*Yes. You don't want to get all from one institute or even just one department.
and pretty strong?
*the stronger, the better.
If no strong recommendation letter writers, should we just ignore EB1B and just go EB2?
*The main point is you yoursself satisfy the USCIS. Refs are very important but your are the first one.
And in which stage we need to reference letters?
*You should your refs along with your I140 application.
Good Luck.