Sienna is named after the Tuscan city of Siena, famous for its medieval architecture and the deep orange-red colour of the local clay — the colour we call "burnt sienna," used as an artist's pigment for over five hundred years.
Sienna entered the U.S. top 1000 in 1995 and has been rising. Today it sits in the U.S.
Sienna 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 Sienna reduce to 8, The Visionary. This is a traditional interpretive system, not a factual claim about the name.