How many letters does the top answer for “separate or apportion into sections” have?

The most common answer is partition (9 letters).

Crossword clue: separate or apportion into sections

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

partition 9 letters wn-def

a vertical structure that divides or separates (as a wall divides one room from another)

zone 4 letters wn-def

a locally circumscribed place characterized by some distinctive features