Is it possible to use the same external HDD for both macOS and Windows? For example, can I use it as backup storage for both systems?
If so, should it have at least 2 partitions?
I am doing this with TWO External HDs so I can guarantee my advice is correct.
It's possible IF you have (third party) software installed that let's MacOS, Windows, or both READ AND WRITE the other filesystem (the driver's that come with Mac OS are READ-ONLY).
Otherwise you will need separate partitions for each OS. If you are using Time Machine I VERY STRONGLY RECOMMEND you give it a dedicated partition regardless of drivers!