I don't think the code tells us. Equipment Rack is used in a few places in the code, and it generally says equipment racks must be grounded. How and what size are not mentioned. I've never understood why a rack with no power connections would need to be grounded. A rack with power connections will most likely have an item bolted to the rack rails which generally will cause the rack to get bonded (I've even got some special rack grounding screws with thread forming threads and teeth under the head to bond painted equipment). With some of the power coating being done, I'm not so sure there is good electrical contact between various structures of a rack. So where to bond to be effective and where are possible hazards?
Most listed racks will have a grounding screw or thread hole. I'd connect that to a ground stud on a PDU if the rack has one. If there is no power cord going to the rack (or it is an ungrounded system), I would think taking a ground from a panel or branch circuit would be acceptable.