I did not have any luck with shielding.

All of my devices are in a pretty small room (15x10), but I put half on one side, half on the other. I arranged the routers AT least 10 feet away from one another (they will interfere as well). So imagine a sqaure room, with routers at the 4 corners. My routers are on non overlapping channels (except for 1, obviously, as there are only three 2.4 ghz non-overlapping channels).

The routers are only about 5-10 feet away from the devices connected to them. Best thing I did was turn down the transaction power to LOW, as I was so close. The router screaming signal to the phones can also cause connection problems.

Finally, I replaced a bunch of devices with 5 GHZ capable ones. These channels have little traffic, and do not overlap. So I have about 60 phones on the 2.4 GHZ spectrum, and 30-40 on the 5ghz spectrum. I have laptops running earnhoney, so I hardwired all of those to get rid of that traffic too.

Cost is the biggest thing with the 5GHZ capable phones, so search around for which ones are capable and try to buy used. I don't foresee a practical limit to how many phones I could have on the 5GHZ spectrum. Buy good routers too, there are plenty of recommendations around here.