How many letters does the top answer for “a tower with a light that gives warning of shoals to passing ships” have?

The most common answer is lighthouse (10 letters).

Crossword clue: a tower with a light that gives warning of shoals to passing ships

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

lighthouse 10 letters wn-def

a tower with a light that gives warning of shoals to passing ships

pharos 6 letters wn-def

a tower with a light that gives warning of shoals to passing ships