The video interview there they state that each layer will hold a single realms worth of players.
The video with Ion that was posted in this thread he states that the servers they’re using today have the technology to hold “many more players” than the ones they used in 2004. Around the 4:40 mark.
What they intend to set caps to they won’t tell us, only that they can have much higher populations after they disable layering than they could in 2004, that the goal is to keep the number of server low to ensure that every classic server has a healthy population, and that they will add servers if needed later.