Over 98% of all documented WannaCry infections were running versions of the Windows 7 operating system. Windows 7 64-bit edition was the worst hit, accounting for more than 60% of infections. The latest tally of computers that have been touched by this worm is 416,989, albeit not all computers have had their files encrypted, as Wanna Cry’s payload has been defanged by a clever British researcher. The worm used an SMB worm that spread on its own to new computers that ran vulnerable SMB services.
Source: https://www.bleepingcomputer.com/news/security/over-98-percent-of-all-wannacry-victims-were-using-windows-7/