Sally is the English diminutive of Sarah — from the Hebrew Sarah ("princess"). Sally Hemings (1773-1835) — enslaved woman of Thomas Jefferson; DNA testing in 1998 confirmed they had at least one child together.
Sally 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 Sally reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.