How many letters does the top answer for “end, especially to reach a final or climactic stage” have?

The most common answer is climax (6 letters).

Crossword clue: end, especially to reach a final or climactic stage

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

climax 6 letters wn-def

the decisive moment in a novel or play

culminate 9 letters wn-def

end, especially to reach a final or climactic stage

a final climactic stage 50% similar → apogee, culmination