Chelsea is from the Old English cealc (chalk) + hȳð (landing place) — referring to the iconic London district. Princess Chelsea — English-American heritage naming.
Chelsea 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 Chelsea reduce to 8, The Visionary. This is a traditional interpretive system, not a factual claim about the name.