How many letters does the top answer for “having eight sides and eight angles.” have?

The most common answer is octagonal (9 letters).

Crossword clue: having eight sides and eight angles.

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

octagonal 9 letters web-def

of or relating to or shaped like an octagon

having four sides and four angles 60% similar → quadrate

a figure having eleven angles and eleven sides. 50% similar → undecagon

a triangle having its sides and angles unequal. 50% similar → scalene