Nephthys (Greek Νέφθυς, from Egyptian Nbt-ḥwt, "Lady of the House") was the Egyptian goddess of mourning, the dead, and the protection of canopic jars.
Nephthys 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 Nephthys reduce to 7, The Seeker. This is a traditional interpretive system, not a factual claim about the name.