The wireless flash control is one-way.. from camera (or commander flash) to slaves. The camera doesn't know how many flashes there are in each group, and the flashes themselves don't know either. They just emit a pre-flash, the camera meters through the lens and then sends out an adjusted flash command with the appropriate power setting in the command.
Short version: You can have as many flashes as you want as slaves and as stated, split them into 2 groups if you want. The reason to split them into groups is if you want to set the power ratio.