Have I invented this statsitic on Theravada enlightenment? A solitary Buddha occurs in one in a million of the human population: please correct me if that's how often the vehicle occurs -- though I struggle to completely understand the concept of a "solitary" seed or fixed yana. Either way, that one million figure is -- I think -- an expression of how it is more unique than arhatship. 

Or, have I invented the statistic in bold? Anyway:


  [a]s of 2016 Thailand had 39,883 wats (temples). Three hundred-ten are
  royal wats, the remainder are private (public). There were 298,580
  Buddhist monks, 264,442 of the Maha Nikaya order and 34,138 of the
  Dhammayuttika Nikaya order. There were 59,587 Buddhist novice monks.


Thailand's population is about seventy million, which works out, if you ignore 'turnover', as one in two hundred being monks.

So that, depending on how you understand the numbers in the first paragraph, suggests in monks, no less than one in a thousand are already arhats. Likewise, assuming they don't get stuck on the first step, in which case more still, about one in a hundred are stream winners.

Is that completely wrong?

Does it make sense, do you know, to say that there are e.g. three thousand sages in Thailand? Three hundred saints?

How does it compare with success in the English speaking nations?
This inference of the amount of attainments seems unsubstantiated. Those Thai monks are also split into factions where one will hold that Buddha lives in Nibbana and be in disagreement with others. All the factions can not be considered as equally valid traditions with right views, likely to produce the four types of contemplatives. There seems to be relatively little consensus on matters of doctrine among the Theravadins.

If you look at buddhist forums you should readily see how much disagreement there is and things aren't much better in the monastic circles afaik. From what i've seen there is usually a demand for conformity of views, at least in the small communities whereas the larger communities will tolerate a broader range. If one is ordained in one tradition one may or may not be considered to have a valid ordination by others.

The assumption that 1:1m can be a Buddha, if that is what you expressed, i don't know how you inferred it but we are about 7billion+ nowadays and the general consensus is that there has only been one Buddha in the last 2600 years. Furthermore nobody is expected to attain Buddhahood any time soon. Based on this alone one could assume that it is far more of a rarity than 1:1m.