Allison comes from the Old French Alison, a diminutive of Alice — itself from the Old Germanic adal, meaning "noble." In medieval France, Alison was a common girls' name; the name crossed to England with the Normans.
Allison rose dramatically in the U.S. in the 1970s and has stayed in the top 100 since then. The double-L spelling is now more common than the original Alison. Three syllables, classical roots, currently in the U.S. top 70.
Allison 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 Allison reduce to 1, The Leader. This is a traditional interpretive system, not a factual claim about the name.