As the title suggests, Google Play does not show how many devices are allowed in one subscription to Norton Mobile Security. Even the product description on Norton by Symantic only says "multiple devices" See my review here on Google+.
Norton still would only reply as follows:
Hi David, we are sorry to hear you are having issues with how many devices can be covered by one subscription of Norton Mobile Security. We can further assist you but require additional information. Please visit and create a post on the Norton Mobile Security Forum at http://norton.com/nms_support.
And, the answer is??