As a poster above noted, it depends on the software. Devonthink and Scrivener , I think, allow multiple installations along the lines of the note below:
From the Devonthink forum:
"It's possible to install the software on multiple computers as long as you're using it only on one at the same time."
I think Scrivener is similar, but their website is currently down, so I can't verify. Except that to use on both Mac and PC, you must have two licenses, one for Mac and one for Windows (which I do).
Wolfram's Mathematica is an example of software that is not allowed on multiple machines. Indeed they are paranoid in their licensing approach.
Microsoft is similar: Office for Mac: One Copy per Device. You may install one copy of the software on one device. That device is the licensed
device.