Most app stores allow you install purchased apps on multiple devices, but impose an upper limit. Windows 8 previously had a 5 device limit, but from October 9, that will rise to 81 devices. (Yes, 81 -- we're assuming that some of kind of Windows 8.1-related inside joke.)
The change was announced in a post on the Microsoft app builder blog:
Since we launched Windows 8, we heard growing feedback from many developers and from our most enthusiastic customers that the limit of 5 was not enough for their needs. Developers asked for more flexibility in implementing their business models, and customers wanted to run those apps on the variety of tablets, laptops and desktops they owned.
App developers who do want a lower limit can impose one through software, but have to clearly disclose that to customers. Note that this still requires you to sign into the relevant device with your own Microsoft account; you can't give paid apps away to other people to use freely.
The five-device limit is fairly common; Apple, for instance, won't allow more than five desktop machines to be authorised for a given iTunes account.
Increasing the app roaming limits [Windows App Builder Blog]