Maximum number of recommendation letters for assistant professor position I am applying for faculty positions as assistant professor. I have two questions regarding recommendation letters. 

(1) Is there is a limit for the number of references? For me, I have many research collaborators who I can ask them to write me a reference letter. Most of faculty positions ask for three reference. Is it a good indicator when more than 3 references sent their letters to the research committee?

(2) Is it okay to add one reference who is currently assistant professor and was my lab mate for two years? We also co-authored many papers together.
When reading references committee members evaluate them based on different evaluation criteria. Some place a lot of weight on the worst opinion voiced. In that respect, having more references than needed is a risky prospect. Another point to note is that you’re never 100% sure that all your referees write the most enthusiastic letter possible. For example, they have a student on the job market at the same time. 

In my experience, additional letters carry little benefit and potentially expose you to a lot of risk.

While this varies widely by discipline (and by department I’d imagine), reference letters (in math/CS at least, but again YMMV) carry more weight when they’re from big shots. In that respect, a letter from a junior collaborator is not going to have as much impact, especially since this is a close collaborator who will most likely say only good things about you.