How many letters does the top answer for “the commander or chief of a thousand men.” have?

The most common answer is chilian (7 letters).

Crossword clue: the commander or chief of a thousand men.

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

chilian 7 letters web-def

Of or pertaining to Chili. -- n. A native or citizen of Chili.

chiliarch 9 letters web-def

The commander or chief of a thousand men.

a captain or commander of ten thousand men. 50% similar → myriarch