Constanza is the Spanish-Italian form of Constance. Constanza Manuel of Castile (1316-1345) was the Castilian princess and Queen of Portugal through her marriage to King Pedro I — her death cleared the way for Pedro's love affair with Inês de Castro. Multiple medieval Iberian queens.
Used widely across Spain and Italy.
Constanza 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 Constanza reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.