Tay is a modern English diminutive — most-commonly short for Taylor (which derives from Old French tailleur — tailor). Tay as a diminutive is most-associated with Taylor Swift (born 1989) — among the most-successful musicians of the 21st century. Fan community widely known as Swifties.
Tay 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 Tay reduce to 1, The Leader. This is a traditional interpretive system, not a factual claim about the name.