Number of Theravada monks

[pilgrim]

Does anyone know of a resource where one can derive the number of monks in traditional Theravada countries over the last several decades? We know of the countries population over this period so the percentage of monks to the population can be used as an indication of the strength of the Sasana in these countries. I believe that despite the overall increase in population, the number of monks have fallen significantly, but it would be good if there are some real numbers to look at.