There are no device limits as such. However, if you don’t have boost you can only watch on one device at a time. When you switch from device 1 to device 2, you need to completely stop the stream by going to the home menu before starting on device 2.
Now for the gotcha. It can take anything up to 15 minutes for the server to notice that you are no longer watching on device 1. You just have to be patient. Or subscribe to boost, which will allow up to 3 streams at once as well as removing 3rd-party ads from on-demand and giving higher resolution.
Er, not quite, see:-
“When you stop watching on your first device, make sure you exit the show or movie you were watching rather than just pressing pause, turning off the TV or closing the app. This will signal that the stream has ended.
If you don't do this, you might need to wait up to 10 minutes before you can start watching on another device.”
i.e. it is possible to move from one device to another almost immediately, but you have to do it in a way that signals to Now that you have stopped watching on the first device.
Otherwise, you have to wait until Now does the rounds and notices this, which is where the possible ten-minute delay before you can watch on device 2 comes in.
So make sure you exit the stream cleanly on device 1, as described above, and you don’t even have to log off before starting device 2 and carrying straight on,
None of this stops getting Boost from being a bloody good idea, of course.