Trinh is the Vietnamese word for "pure" or "virginal" — from the same root as the Trinh dynasty (1545-1787) which ruled northern Vietnam. Common as a feminine name.
Trinh is widely used in Vietnam.
Trinh 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 Trinh reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.