What's the known figures for the world population of magical humans? Further details, where available, is appreciated - population levels across geographical regions etc.
We don't know.
There are apparently sufficient wizards to merit eleven wizarding schools around the world (typically one or two per continent). Assuming student numbers at those schools are much the same as a Hogwarts (600 students in total, with approx 40 new students each year) and with wizards living an average of around 100 years, that gives a total worldwide magical population of around half a million wizards and witches.
That being said, JKR's maths is appalling and it's not entirely clear that she gave any serious thought to worldbuilding so these numbers (riddled as they are with my assumptions) may prove to be dramatically wrong.