Reagan comes from the Irish surname Ó Riagáin, from rí (king) plus a diminutive ending — meaning "little king" or "descendant of the king." It was traditionally a male surname before becoming a unisex given name in the late 20th century.
U.S. Today Reagan sits in the U.S. top 90, used about 70% for girls.
Reagan 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 Reagan reduce to 1, The Leader. This is a traditional interpretive system, not a factual claim about the name.