Scarlet is the single-T spelling — from the Old French escarlate (the brilliant red dye and the cloth dyed with it), itself ultimately from a Persian word. The 1839 Crayola crayon "Scarlet" has been one of the longest-continuous crayon colors.
Scarlet 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 Scarlet reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.