If you are in America, most rack of lamb represents 8 chops, and is enough for 2-3 people if one of the people isn't very hungry. If the rack has been frenched - that is the bones were scraped down, then you really want to allow for 3-4 chops per person. Some of it can also depend on the size of the rack itself, and if you are getting one from a spring lamb which is small versus a rack from a lamb closer to a year old, which would obviously be larger. I usually estimate 1-2 ounces per rib chop, and 4 chops per person.
If you have Costco, go there.