How many letters does the top answer for “the mark (') used to indicate the omission of one or more letters from a printed word” have?

The most common answer is apostrophe (10 letters).

Crossword clue: the mark (') used to indicate the omission of one or more letters from a printed word

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

apostrophe 10 letters wn-def

the mark (') used to indicate the omission of one or more letters from a printed word