Collaborators are OK and easy to ask for. Mine ones are all collaborators. Based on my experience, independent recomenders are not that important. One or two is enough.
Posdocs or scientists in national lab or government are better than none. Be sure to ask 3-5.
Letters can be as many as 10.
I had 11 letters (many "somebadys") and got I-140 approved in 17 day.
Good Luck!