Steffi is the German diminutive of Stefanie — from the Greek stephanos (crown, garland). International Tennis Hall of Fame (2004). Married Andre Agassi in 2001.
Steffi 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 Steffi reduce to 2, The Peacemaker. This is a traditional interpretive system, not a factual claim about the name.