Brianna is the English feminine of Brian — from the Old Irish Brígh ("noble, high, strong") — first popularized in Edmund Spenser's The Faerie Queene (1590).
Brianna 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 Brianna reduce to 5, The Seeker. This is a traditional interpretive system, not a factual claim about the name.