How many letters does the top answer for “arranged in three vertical or spiral rows.” have?

The most common answer is triserial (9 letters).

Crossword clue: arranged in three vertical or spiral rows.

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

triserial 9 letters web-def

Arranged in three vertical or spiral rows.

triseriate 10 letters web-def

Arranged in three vertical or spiral rows.

arranged in three vertical rows. 80% similar → tristichous