If i make the wall three blocks thick, and make water on the ceiling, would I be able to contain many endermen in a giant end stone room? I am doing this for an adventure map so I need the answer for the final boss battle.
Endermen will teleport to any block within 32 blocks in each axis. While it will not spawn or teleport on\to blocks that has lava or water on them. This means you can't really contain them to one smaller area. It's not unusual that they can teleport multiple times quickly.
In other words, unless you use a mod, you can't really contain it. Only way I can think of without mods is making a massive, flat area with loads of water\lava and no real places for it to teleport. It can't spawn nor teleport to\in areas 2 blocks or lower.
Technically yes, but they might teleport away, since they randomly do that.
Endermen always initially spawn in pairs, but will often appear individually or in larger groups due to random teleportation.
Your best bet would be to simply spawn them in using a clock and command block. While your setup should keep most of them in the area, they can teleport away randomly (as Dylan noted). In Vanilla, that's about as good as you can get.
If you are able to install the Extra Utilities mod, as well as one of the various mods that allows you to create helper NPCs with melee capabilities, then you could have a turtle hit them with "The Ender" sword as they spawn, which prevents teleportation on the first hit - but would weaken each by about 6 - 8 health.
Best advice, make sure there's no fire or water in the space you intend to spawn them, because wandering into either is going to cause them to instantly teleport away. Just keep re-stocking them with a redstone clock for the duration that you need to.