Usually I was doing only one mesh per fbx file.
Recently I started to shape ground from small patches and I decided to put all patches types in one file since it was easier to export all patches at the same time this way.
For me, big standalone files get their own blender file. Characters, big buildings, etc. Which I imagine is standard stuff.
But if I’m working on stuff like… rocks, or trees, etc, then I usually have blender files to group them accordingly. I do this more and more as I get deeper into geometry nodes too.