This is a findable answer[0], though this only shows 3 or more children. Looks like 7,008,000 families have 3 or more children. Looking at the same site[1] for the number of people between, say, 15 and 49 (the thought being anyone less than 15 probably isn't a "parent" and anyone over 49 doesn't have children living at home anymore, or at least not ones they need to transport), we get 150.67 million "parents".

Some quick math gets us about 4.6%. Incidentally, that's similar to the number of LGBTQ persons in the US (4.5%)[2]

[0] https://www.statista.com/statistics/183790/number-of-familie...

[1] https://www.statista.com/statistics/241488/population-of-the...

[2] https://news.gallup.com/poll/234863/estimate-lgbt-population...