If all it takes to be a "Karen" is to be a middle aged, white, woman, then perhaps 25% of the US would be Karens. Maybe more. Anyone making that assumption is clearly misusing or misunderstanding the term, perhaps intentionally.
I've seen the term used in two main contexts:
1. When "Karen" is using her status as a "wealthy" consumer to bully a low-wage employee for her own benefit. The stereotypical example is Karen demanding a free meal in a McDonalds because her Big Mac had too many pickles on it, or something of that nature.
2. When "Karen" calls the police on a non-white individual for doing something routine, incorrectly reporting it as suspicious.
Karen #1 is a bully; Karen #2 is a bigot. The state of California is considering a bill that they called the CAREN Act that actually criminalizes action #2.
Until a couple weeks ago, I had only heard the term used in the first sense, but apparently both uses are fairly common. I am not aware of the widespread use of the term to describe ALL white women of a certain age.