Actually there are precise mathematical theorems exactly about which degree and mode of aggregation will give you which degree of reliability in the aggregated result. This is in large part what mathematical statistics is about.
The thing with mathematical theorems is that they rely on assumptions. As long as we are in the gambling, boardgaming, card drawing world, mathematical assumptions can usually be checked precisely (i.e. they are true or not). As soon as you start real world statistics, you often have to make educated guesses if certain assumptions are met or if slight deviations from assumptions are still ok enough.