A big proportion of everyone who uses a non-Apple keyboard with their Mac, I'd say. I would absolutely use one with my ergonomic keyboard, TouchID is so much more convenient than constantly typing your password in.
If you were to guess how many sales (e.g. in units) they'd make every year, what would you guess?
I jest, but probably the $150 sales of the TouchID keyboard would tank as they are displaced by such a device.