The best way to look at an object that's going to be printed is to think in horizontal layers. If you look at a model ata certin height and there are parts to print which aren't attached to the rest of the model - you'll need supports or print at a different orientation.

And shells should pretty much always be a minimum of two. A shell is the outline of a model before it starts the infill. So it's essentially the bit that shapes and details what you are printing.

Adding extra shells will also make a print much stronger without increasing infill.