I believe that Blue Tooth supports up to 7 distinct channels of communication between devices but that doesn't (necessarily) mean that you could only have 7 Wiimotes working on a system ... Hypothetically speaking, you could have multiple devices communicating on a single channel if there is a way to seperate the communication. To keep this as non technical as I can, for this to work you would need a prefix on each communication between the Wii and the Wiimote in a channel. Basically the technical limitation on number of controllers would be:
7 x ( Bandwith of Channel / Bandwith needed by Wiimote)
In reality though, it is likely that Nintendo limited the Wii to handling 4 Wiimotes and is keeping the other 3 channels open to Add-Ons like Microphones/headsets.
