Placidia is the Latin feminine from placidus (calm, gentle). Galla Placidia (388-450 CE) was the half-sister of Emperor Honorius — captured by Visigoths and married their king Athaulf, later returning to rule the Western Roman Empire as regent for her son Valentinian III.
Placidia 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 Placidia reduce to 1, The Leader. This is a traditional interpretive system, not a factual claim about the name.