Well, according to the US Census Bureau's American Community Survey, about 78% of Americans fit into the Middle Class if we define that as a household earning a median household income between $15,000 and $149,000 per year. Another 12% of Americans earn less than $15,000 a year. And if you and your household earn more than $150,000 a year, you belong to the top 10% of household earners and thus the Upper Class.
If you are smack dab in the middle of the Middle Class, your household earns around $53,000, the median household income.
Median income, however, is on a continuum. So it is subjective how we define Lower, Middle, and Upper Classes of people. Moreover, sociologists find that what class people identify with has far more complexity than simply how much money one earns.
In fact, recently the Pew Research Center found that the percentage of Americans who identify as Middle Class has dropped to the point that almost as many Americans now identify as Lower Class. Americans identifying as Upper Class have also declined since the Great Recession hit at the end of the Bush administration.
Thus, no majority of Americans identify with any social class since around 2011. Instead, the plurality still identifies as Middle Class but just barely.