Question about Synology DS120j

In principle, yes, see software specifications --> Surveillance Station https://www.synology.com/de-de...

Of course, it depends on the resolution and video codecs of the cameras. But if motion detection is also done on the DS, it might not be much fun any more.

You can compare several DS models under the following link, where you can see the differences in performance very well https://www.synology.com/de-de....
if you want to compile a comparison page yourself --> https://www.synology.com/de-de...