Thembi is from the South African Zulu / Xhosa thembi — from thembekile (trustworthy, faithful) or ithemba (hope). Thembi is one of the South African feminine names — central to traditional Zulu + Xhosa cultural heritage.
Thembi 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 Thembi reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.