Plath is from the German Platt (flat) — historically a surname for one who lived by a flat field. Aurelia Schober Plath (1906-1994) — Sylvia's mother.
Plath 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 Plath reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.