If all letters are strongly

supportive and from highly authorative experts in your field, you can keep all. Otherwise, it is advisable to drop a few. Most attorneys recommend using letters from independent experts besides your current boss, "independent" meaning NOT having actually worked with or personally acquainted with you.

One thing to be careful of is that university professors tend to treat immigration rec letters as seriously as professional rec letters. In that case, unless you are really really exceptional in research and your work is really really important in benefiting the nation, those highly objective (i.e. non-exaggerating) letters could actually hurt your chance of passing NIW. This happened to a friend of mine.

Good luck.