First off, multiple pieces requires multiple mendings, it's true. Second, the break doesn't have to actually divide the object in two, otherwise it couldn't fix, say, a rip in the sleeve of a shirt, or a hole left from stabbing through a tent wall. So you repair the phone screens. One crack = 1 break.

You could try to be pedantic and say a dropped phone could arguably have hundreds of little cracks, but the same pedantism applies to any cut in fabric, it's actually hundreds of individually cut threads. There's a point where you have to stop lumping tiny things into one "break", obviously, but that point is when lumping them together creates a break that's more than a foot across.

Fixing phone screens should be valid with mending. Of course, phones are more than just screens, so you might need multiple Mendings to fully fix a phone, but you'll do great at screen repair at a minimum.