The census bureau doesn't directly define "homemaker," but it does keep statistics that come close to answering the question of how many American women are homemakers?
There are about 90 million American women aged 20-64, of whom 29% are not in the workforce, i.e. not employed and not looking for paid employment (about 17% of men the same age are not in the workforce, a status that includes students, early retirees, prison inmates and people discouraged from looking for work). People with lower levels of educations are much more likely to be out of the workforce than those with higher levels of education.
Typically, adults age sixty-five or older who are not in the workforce are considered "retired" rather than to be "homemakers" regardless of their prior employment history. Students and pre-school children are also not considered to be in the workforce. The number of women aged 20-64 is overinclusive, because it includes, for example, adults living with their parents, or alone, who have become discouraged in their search for work, perhaps due to a disability, older college students, and early retirees. It may also include institutionalized adults in prison. But, it is also underinclusive, as it excludes women under the age of twenty who are not in the workforce such as teen parents living with a significant other, who would also generally be considered to be homemakers.
Still, for an order of magnitude number, this is close enough to peg the number of American women who are homemakers and not employed in paid work at something on the order of 15-25 million American women. While this is far less than a majority of adult American women, defying 1950s stereotypes (notably 64% of women with children under age six at home have paid employment), it is also probably still the most common occupation of adult, pre-retirement age American women.
There are about 300 million Americans overall across all ages and genders.