Arwen is a name invented by J.R.R. The name's US use began modestly after the films and has grown steadily.
Arwen 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 Arwen reduce to 7, The Seeker. This is a traditional interpretive system, not a factual claim about the name.