What law sets the minimum population requirement for an American state?

The Northwest Ordinance, which requires a minimum of 60000 people to form a state.

https://en.wikipedia.org/wiki/Northwest_Ordinance

"Admission of new states
The most significant intended purpose of this legislation was its mandate for the creation of new states from the region, once a population of 60,000 had been achieved within a particular territory."

I believe that The Northwest Ordinance applied only to the states that would be formed out of the Northwest territory. It stated that states would be formed "So soon as there shall be five thousand free male inhabitants of full age in the district,"
http://avalon.law.yale.edu/18th_century/nworder.asp

In the late 1850s, when Congress resubmitted the Lecompton Constitution to Kansas voters, Congress stipulated that if the voters rejected it, Kansas would have to wait until its population reached 93,420 before admission.
teachingamericanhistory.org/library/index.asp?document=102 no longer exists
https://www.battlefields.org/learn/articles/lecompton-constitution

While neither of these pieces of information directly answers your question, they seem to indicate that population requirements may have varied from territory to territory and that there is/was not one single law covering every single state.