Leto (Λητώ) is from the Lycian lada (woman, wife) — the Greek Titaness daughter of Coeus and Phoebe. Artemis was born first and immediately helped her mother deliver Apollo — for which Artemis became the goddess of childbirth.
Leto 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 Leto reduce to 7, The Seeker. This is a traditional interpretive system, not a factual claim about the name.