How many letters does the top answer for “make copies on a roneograph” have?

The most common answer is roneo (5 letters).

Crossword clue: make copies on a roneograph

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

roneo 5 letters wn-def

a rotary duplicator that uses a stencil through which ink is pressed (trade mark Roneo)

make a copy or equivalent of 50% similar → reproduce