Sasha is the Russian diminutive of Alexander/Alexandra — from the Greek alexandros, meaning "defender of mankind." In Russia and other Slavic countries, Sasha is used as an everyday name for people named Alexander or Alexandra of any age.
In English-speaking countries, Sasha is used almost exclusively for girls. Sasha Obama (born 2001), daughter of President Barack Obama, is a prominent bearer.
Sasha 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.
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In Pythagorean numerology the letters of Sasha reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.