It's the same combinatorial explosion as with the future possible worlds. Even though you can't locate individual valuable future outcomes (through certain instrumental sequences of exact events), you can still make decisions about your actions leading to certain consequences "in bulk", and I expect the trade between possible worlds can be described similarly (after all, it does work on exactly the same decision-making algorithm). Thus, you usually won't know who are you trading with, exactly, but on the net estimate that your actions are in the right direction.