Mencía is from the Basque mendi (mountain). Mencía López de Haro (c. 1215-1270) was the Castilian noblewoman who became Queen of Portugal through her marriage to Sancho II. The name was widely used among medieval Castilian nobility.
Mencía 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 Mencía reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.