Plug in $12,000 as the annual income, and that'll tell you that 24% of U.S. workers made that or less in 2014.
Looks like the number of full time U.S. workers in 2014 was 118.72 million: https://www.statista.com/statistics/192361/unadjusted-monthl...
That works out to be 28 million U.S. workers making <= $1,000 per month. Considering that's excluding part time workers, that should be make the calculation fairly conservative.
See also: https://howmuch.net/articles/how-much-americans-make-in-wage...
The specific numbers have changed in 6 years, but I don't find it all that unreasonable to suspect that there are many millions of U.S. citizens making less than $1,000 per month.