Belle is from the French belle (beautiful) — the feminine form of beau. "La Belle" — "Beauty" — is the heroine of the French fairy tale La Belle et la Bête (Beauty and the Beast) first published by Gabrielle-Suzanne Barbot de Villeneuve in 1740; Jeanne-Marie Leprince de Beaumont's 1756 abridged version is the version most commonly read today. Disney's animated Beauty and the Beast (1991) was the first animated film nominated for Best Picture at the Academy Awards; the live-action 2017 remake with Emma Watson became one of the highest-grossing films of 2017. Belle Starr (1848-1889) — American outlaw.
Belle 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 Belle reduce to 9, The Giver. This is a traditional interpretive system, not a factual claim about the name.