The version of Windows following 8.1 will be Windows 10, not Windows 9. Apparently this is because Microsoft knows that a lot of software naively looks at the first digit of the version number, concluding that it must be Windows 95 or Windows 98 if it starts with 9.
Many think this is stupid. They say that Microsoft should call the next version Windows 9, and if somebody’s dumb code breaks, it’s their own fault.
People who think that way aren’t billionaires. Microsoft got where it is, in part, because they have enough business savvy to take responsibility for problems that are not their fault but that would be perceived as being their fault.