how many times can rotors be turned or machined? on a 2002 f150.

I would recommend once. Every time you turn them you are taking metal away causing them to warp quicker each time.

I often see rotors that can't even be machined the first time. Depends on the type of pad, how severe the duty, and what you had to start with in the first place (OEM isn't always the best). Your truck may require entire hub/rotor assemblies. If that's the case, I recommend spending a little more money (initially) and picking up the kit that's hub and rotors separately. NAPA carries them. Not cheap but cheaper in the long run.

It all has to do with the thickness of the rotor. There is a minimum thickness stamped somewhere on the rotor. Once it is below that no reputable shop will turn it and it would not be safe to either. The good news is if your rotors look to be in good shape and are not warped it is not necessary to turn them when installing new pads.