Elvira is from the Old Germanic Gailwira — gail ("happy, joyful") + wer ("truth") — popularized in Spain through the Visigothic queens; later embraced by Spanish opera tradition. 21 became forever linked to it.
Elvira 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 Elvira reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.