I doubt you can generalize globally given the variability in what tasks, for example, dialysis nurses are empowered to do in different countries and clinic groups. Transplant infrastructure and geographic dispersion of offices/clinics/centers would be key variables as well. And, of course, availability of organs...I'd work from the now, looking at actual numbers of nephs, the countries/provinces/cities they are in, and the characteristics of those geographic units of analysis, then try to impute to wherever you are looking to optimize or report.
In my opinion there should not be any such guidelines because in many parts of the world we have not even achieved the required number of medical graduates per population as prescribed by WHO.
There is no such population based study till now up to the best of my knowledge