How many letters does the top answer for “occurring twice a week” have?

The most common answer is biweekly (8 letters).

Crossword clue: occurring twice a week

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

biweekly 8 letters wn-def

a periodical that is published twice a week or every two weeks (either 104 or 26 issues per year)

semiweekly 10 letters wn-def

a periodical that is published twice each week (or 104 issues per year)

occurring twice a month 50% similar → semimonthly