The general rulre of thumb is to have 3 LORs minimum. 1 from a non-science prof, and 2 others from science profs. But like i said, it depends on the school. Some schools only require 2 LORs. Some schools require 3. Some schools don't care if one of them is non-science or not. Most schools don't want more than 5 LORs total. Just depends on the school.