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

The most common answer is july (4 letters).

Crossword clue: the seventh 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).

july 4 letters web-def

the month following June and preceding August