FastFar is licensed on a per platform basis. What this means is that PC, Mac, iOS, and Android, are all separate purchases. This is so because Google and Apple rules require this - we as app creators have no say in the matter.
If you have multiple PCs that you'd like to use FastFAR on, you may do so with a single license, within reason. The one caveat to this is that it must be you and only you who is using it, as it's sold on a single-user license basis. If a friend or colleague would benefit from using FastFAR, please direct them to purchase their own copy.
By "within reason" we mean usually that a single user might use FastFAR at home, on a laptop, and perhaps on a work PC. If you get a new PC, then you can install your licensed copy there.
For Mac, iOS, and Android - please refer to the terms of license provided by Apple and Google at the time of sale. However, in general, you will be able to install the apps on multiple devices of the same type (for example, 2 macs or an iPad and an iPhone) connected to the same app store account. Please note that in the Apple universe, the mac and iOS app stores are entirely separate.