How many letters does the top answer for “author of the twelve chairs” have?

The most common answer is brooks (6 letters).

Crossword clue: author of the twelve chairs

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

brooks 6 letters wd-fact-v3

United States literary critic and historian (1886-1963)

ilf 3 letters wd-fact-v3

author of the chairs 67% similar → ionesco

wrote the twelve chairs 50% similar → brooks, ilf