Dax is from the French place name Dax (a southwestern French spa town) — possibly also a modern American invented name. Dax Riggs — American singer (Acid Bath). Dax-Devlon Ross — modern American figure.
Dax 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 Dax reduce to 2, The Peacemaker. This is a traditional interpretive system, not a factual claim about the name.