I don't look at it in terms of a strict number, but this is my view on the subject.
Those who are short have a much greater benefit doing cosmetic lengthening than those who are average height. Short people doing lengthening to become average height or at least less short than they were originally have not only a psychological benefit of doing the procedure, but also a benefit in escaping the discrimination that many short men face. For them, there's a good deal of worth in risking the many potential complications of the lengthening procedure. At the same time, those who are of average height doing cosmetic lengthening to get the benefits of being tall aren't escaping the same social discrimination, rather they're just potentially getting the added social benefit of being a tall person. In that case, I see it as too much risk for such little payoff.
My perspective is probably biased coming from the former category, but when I see people who are average height wanting to get lengthening so they can be one of the privileged tallers, I can't help but think they may have a bit of an ego problem and would do better getting therapy than potentially crippling themselves through cosmetic lengthening.
Also when I'm saying 'short' and 'tall' I'm talking about what's considered short or tall in the person's country of origin, because a person who is short in the Netherlands could be considered tall in many places around the world, like Asia or Central and South America.