The maximum amount of resources you can raid during multiplayer battles are 50% of resources stored in the gold/elixir collectors, 75% of dark elixir stored in dark elixir drills, 5% of dark elixir stored in dark elixir storage, plus 25% of gold/elixir storage up to a maximum of 199K, and 100% of what is stored in the Town Hall. However, if the opponent has a Town Hall which is 2 levels lower than yours the maximum available loot is reduced by 50%. 3 or more levels lower and it's a 75% reduction.