Are there any solid figures for how many Roma there are, worldwide, and how they are dispersed?

When I was a kid(40s) there were still stories of abduction and theft and unaccountability of and by "gypsies" but if memory serves me, when it came to unacceptable practices within the cities, the "Williamson Gang" was much more commonly spoken of. I suppose it is possible that the 'Williamsons' were Roma.