I can tell you that ARM and MIPS ship about 5bn 32-bit MCUs per year combined. I can't give you a statistic for 8- or 16-bit MCUs though.
That said, trying just to understand how many 8051 derivatives is almost impossible. There are many dozens of vendors who supply chips, cores & other embedded IP, many of whom don't even call it 8051 any more. There's probably a few tens of them within 100 feet of you. Much less the 20 or so other significant 8-bit families.
It's a shame that hard data is only available from high dollar market research firms, but I suppose they did the legwork and deserve something for the work.