The official statement is that you can have six activated devices (for me: 2x 360, PS3, 2x iPad, iPhone). PCs don't count and activating a new device knocks the oldest device off the activation list. Haven't used your Roku in six months, but got a new iPad, TV, phone, and Wii U? The Roku will need activating when you come back to it.
You also get two simultaneous streams with the standard $8 streaming-only service. In practice this can be three or four streams at once, but they will lock it down to two if they find you abusing it. They don't want multiple households sharing a single account, but they also don't want to punish reasonable watching within a household. If you also subscribe to discs, then you get as many simultaneous streams as discs. 4-Disc plan? 4 streams.