Polly is an English diminutive of Molly — itself a diminutive of Mary (from the Hebrew Miriam).
Polly was hugely popular in 18th and 19th-century England. Today Polly is returning fast as part of the vintage revival.
Polly 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 Polly reduce to 8, The Visionary. This is a traditional interpretive system, not a factual claim about the name.