https://www.rand.org/blog/2021/05/most-americans-consider-themselves-middle-class-but.html
39% of people describe themselves as lower class, 47% middle class, 12% upper class.
However,
"A majority of millionaires polled describe themselves as middle class or upper middle class despite being among the wealthiest 10 percent of Americans, according to the results of the third CNBC Millionaire Survey.
Fully 44 percent described themselves as middle class, and 40 percent said they were upper middle class. Only 4 percent described themselves as wealthy or rich, and 5 percent described themselves as upper class."
https://www.cnbc.com/2015/05/06/naires-say-theyre-middle-class.html
So if given the option, a lot of those upper class people would describe themselves as upper middle.
I agree that it's all relative. Relative to the world, I'm rich. Relative to the US, maybe rich (we're perhaps 95th percentile income). In the DC area, upper-middle class. In our neighborhood, average.