How many letters does the top answer for “combine or increase by multiplication” have?

The most common answer is manifold (8 letters).

Crossword clue: combine or increase by multiplication

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

manifold 8 letters wn-def

a pipe that has several lateral outlets to or from other pipes