Ella has two origins. In Germanic languages it is a short form of names beginning with alja- (other, foreign) — names like Alia or Albertina. In Hebrew, Ella (אֵלָה) is a tree name, specifically the terebinth or oak, and appears as a place name in the Hebrew Bible.
Ella has been continuously used in English from the medieval period, popularised by Norman families. It sits in the U.S. top 25.
Ella 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 Ella reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.