How many letters does the top answer for “denoting a quantity consisting of six items or units” have?

The most common answer is vi (2 letters).

Crossword clue: denoting a quantity consisting of six items or units

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

vi 2 letters wn-def

more than 130 southeastern Virgin Islands; a dependent territory of the United States

denoting a quantity consisting of 12 items or units 83% similar → dozen, twelve, xii