Is it OK to install one Office 2019 license on both Mac and Windows? I have purchased a standalone version of Office 2019 for my Mac. So far so good. I also have a spare Windows 10 notebook and would like to occasionally run Office apps on it (not at the same time as my Office 2019 Mac version). 

Does the license key allow such an installation scenario?
No.

According to Microsoft's Office product page, it is a one time purchase for 1 Mac or PC



This means that you can only install it on 1 machine - either a Mac or a PC, but not both.

There's also an excellent comparison chart showing what's covered between the 2019 standalone version and the O365 subscriptions.  As for number of devices, only the O365 subscriptions allow for multiple device installations.