You can’t any more.
But except for tidiness, you don’t need to, as when you open a seventh device, this just pushes the device with the oldest usage, and no downloaded programmes, off the list to make room for itself.
Are you currently watching NOW on one of the devices?
Do you have boost?
That’s a slightly different question, but one I can answer also.
You don’t need to delete any devices, just to log off Now on them.
You are now limited to watching on one device at a time, unless you buy the Boost, when you can watch on three devices simultaneously. Boost also gives you HD instead of 720p, 5.1 surround instead of just stereo, higher refresh rate for Sports, and freedom from third-party adverts, so it’s well worth considering.
Actually, you may not need to log off Now, just to come out of the programme you are in, or to switch away to another app. But from now, sort it by logging out, to be sure.