How many installations are allowed with a single subscription to the Pro version? For example, can I install it on both my desktop and laptop at home?
Hello, welcome to the Community! 
Yes, you can install Papyrus Author on a second computer. The installer can be found on the Downloads Page.
Note that your documents are not saved inside of your Papyrus Author account or the software itself, but on the computer’s hard drive. The default saving location is in your operating system’s user folder. If you’re searching for documents, you can identify Papyrus Author files with their .pap extensions.
When working on multiple computers, you can move your projects on a USB drive, or set up a cloud path on Papyrus Author. Here is a tutorial video on it: Backups: Saving with Incremental & Automatic Backups