Thanh is from the Vietnamese Thanh (清 / 青 — pure, clear, peaceful, blue-green). Thanh is one of the Vietnamese feminine + unisex names — central to traditional Vietnamese cultural heritage.
Thanh 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 Thanh reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.