The Relative Clause is “who are five in number”. It is a type of dependant clause (it need the main clause in order to convey its meaning properly) that has the same function of an adjective; that is to say, it modifies a noun “friends”.
In terms of concussion rates per 1000 exposures, the girls' high school sports with the highest rates of concussions are soccer (0.36), basketball (up to 0.21), and lacrosse (0.20).
