There is no limit on the number of iPads you can install StaffPad on once you've purchased it from the App Store. Of course, all these iPads must be signed in with the same Apple ID.
You can also set up Family Sharing to share StaffPad with other members of your household. Please note, however, that any in-app purchases are not shared with the rest of the members in your household. For more information, see this link: https://support.apple.com/en-us/HT201088
You can download and install StaffPad on up to 10 devices, as long as those devices are all linked to your Microsoft Account.
With the apps in the Windows Store, the apps you buy are linked to your Microsoft Account, rather than the device itself. The same is true of any music, films, games, books etc that you purchase through the Store app. If you sign in with your account on other devices, you should be able to open the Microsoft Store app from there, click your account picture and access your app "Library". From there, you can easily download the app again.
In Microsoft Store, select See more > My Library. Select the app you want to install, and then select Install.