How many letters does the top answer for “the eighth month of the year, containing thirty-one days.” have?

The most common answer is august (6 letters).

Crossword clue: the eighth month of the year, containing thirty-one days.

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

august 6 letters web-def

the month following July and preceding September