Autumn is from the Latin autumnus — the season of harvest and the turning of leaves. (2005-2007), Entourage (2007-2009), countless Hallmark Channel films. Autumn appears across modern literature including Lauren DeStefano's Wither trilogy and Ali Smith's Autumn (2016) — the first novel of her Seasonal Quartet, shortlisted for the Man Booker Prize.
Autumn does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Autumn reduce to 9, The Giver. This is a traditional interpretive system, not a factual claim about the name.