Hard numbers are hard to come by. The Economist gives a number only for South Africa (see this post). The two books reviewed on this site give a few numbers but never a complete coverage. The World Bank put some numbers together for around the year 2000, before the last wave of immigration. I just found some data put together by Park in 2009 that seems to be the best currently available. The map below shows their distribution across countries. The scale is in log.