Does dark roast have more caffeine than light roast?

No — it's a myth. Measured by weight (scoops), light roast has slightly more caffeine because the beans are denser. Measured by volume, dark roast may have marginally more because the beans are larger and you fit fewer in a scoop. The difference is negligible either way (less than 5%). Brewing method and coffee-to-water ratio affect caffeine far more than roast level.