It is per "taxpayer" (person).
"The credit allowed under this section with respect to any taxpayer for any taxable year shall not exceed the excess (if any) of $500 over the aggregate credits allowed under this section with respect to such taxpayer for all prior taxable years ending after December 31, 2005."
<a rel="nofollow" target="_blank" href="
https://www.law.cornell.edu/uscode/text/26/25C">https://www.law.cornell.edu/uscode/text/26/25C</a>
