How many letters does the top answer for “the sum or number of seven.” have?

The most common answer is heptade (7 letters).

Crossword clue: the sum or number of seven.

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

heptade 7 letters web-def

The sum or number of seven.

the cardinal number that is the sum of seven and one 60% similar → octad, octonary, ogdoad

the number seven. holinshed. 50% similar → septenary