How many letters does the top answer for “being or denoting a numerical order in a series” have?

The most common answer is ordinal (7 letters).

Crossword clue: being or denoting a numerical order in a series

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

ordinal 7 letters wn-def

the number designating place in an ordered sequence