Most of us tend to believe that the darker the roast, the more caffeine content it might have. However, there are also some people who believe that the longer the coffee undergoes roasting, the more caffeine molecules burn off.
Apparently, both of these assumptions aren't quite true. A dark-roasted bean has the same amount of caffeine as a light-roasted bean. The answer to your question actually depends on how you measure the beans for your brew.
If you're measuring by scoops for brewing, light roasted coffee will most likely have more caffeine because they're actually denser than dark roasted beans. However if you're weighing the coffee beans you will use, dark roasted beans generally have more caffeine content since there will be less mass.