Adela is from the Old German adal (noble). Adela of Normandy (c. 1067-1137) was the daughter of William the Conqueror and Matilda of Flanders — Countess of Blois, mother of King Stephen of England, and important patron of the Cluniac reform.
Adela Rogers St. Johns was the iconic American journalist (1894-1988).
Adela 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 Adela reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.