That is correct. If your watching your roku and try to watch something on the pc, it wont let you, and vice versa with only 1 at a time.
2 dvd's get you 2 watch instantly at a time and the more you up, the more you can watch at the same time (I have the 2 dvd plan and can watch one on the pc and one on the roku, or one on my xbox and one on the roku or etc.. at the same time). Keep in mind you also have to take your bandwidth into consideration, you dont want to get more than your bandwidth will allow.
You can have up to 6 devices registered even on 1 at a time dvd plan, you just will only be able to watch on one of them at a time unless you up the plan.
Another thing about pc's. they drop off the device list after 30 days (unless they have changed this since late last year) of non use (your other devices go in 1 year increments, unless they have changed that to). so if you plan on buying many devices, and do plan on watching instant watch on your pc, that you shouldn't go above 5 so you can save one for your pc.