One night float doing a month rotation (early in the year a 3 or a 4, later in the year a 2 or 3) covers the trauma service (varies but I have seen it as high as 40 or so), responds to traumas, and supervises the on call intern.
One intern on call-covers floor calls and consults for general surg, plastics, urology, ENT (in house census usually around 20, but varies), responds to traumas.
Back up chief-home call (later in the year a 4 takes more of this), gets called in when a case goes to OR at night (night float usually does the case, while the chief covers for them), theoretically will be called in if things are too busy but I have never seen it happen even when things are hectic, gets called if there are questions.