Though archery had always been a sport in Númenor, the King's Archers as a military unit did not emerge until the later Second Age, when the Númenóreans began to war with the Men of Middle-earth. We're not given a date for their formation, but we know that the oppression of the peoples of Middle-earth began with King Tar-Ciryatan (who ruled II 1869 - II 2029) and worsened under his successor Tar-Atanamir (ruled II 2029 - II 2251). It was about this time, then, that the King's Archers would first have been seen in Middle-earth.

The Dúnedain continued to use archers in warfare after the Downfall, but the King's Archers were formally named for the King of Númenor, and so would have ceased to exist when that line came to an end.

We're told little of the organisation or numbers of the King's Archers, other than the use of the word 'cohorts' (ibid). In the Roman army, a cohort (Latin cohors) would classically have contained about five hundred men, though this number varied according to period or circumstances. It's unclear exactly how applicable this would be to the armies of the Númenóreans, but it seems safe to imagine the King's Archers as being organised into companies of at least several hundred bowmen each.