avatÄre á¹£oá¸aÅ›ame
paÅ›yan brahma-druho ná¹›pÄn
triḥ-sapta-kṛtvaḥ kupito
niḥ-ká¹£atrÄm akaron mahÄ«m
The Lord, appearing as ParÄÅ›urÄma, the sixteenth avatÄra, seeing the kings harassing the brÄhamaṇas, became angry and twenty-one times annihilated the ká¹£atriyas from the earth.
Trih-sapta-krtvah means twenty-one times (3x7). Krtvah is used as a suffix after numerals to indicate counting of repeated actions. Also numerals with the suffix suc such as trih indicate repeated actions. (Panini 5.4.17-18)
