The first limiting factor is that the UI only offers four slots for players to connect to. So, the only way to reach the maximum of 8 controllers is by pairing 4 dual-Joycons pairs and then handing one Joycon to each of the 8 players.

if you wanted to use a dual-Joycon setup per player, or use 8 Pro Controllers, then first of all the UI would need to be updated, and then you would be bumping up against the 7-device limit of Bluetooth 4.1. We'll have to see if any games ever attempt to support this and how it ends up working. 8-player gameplay in Smash for example would work in docked mode if you attached wired GameCube or Pro Controllers to the dock.