Why so many SetPass Calls?

The SetPass Calls (or draw calls) are performed per object in your scene. It doesn't really matter whether the objects share the same shader or not. Each separate object in your scene requests its own draw call.

But why do you have such a strange count on draw calls?

Dynamic Batching does what it can to be consistent and not batch unnecessary objects together. This isn't always consistent, however. In some cases, objects may be batched in obvious ways and in other cases, they aren't. It's simply Unity doing what it can to decrease the number of draw calls when it seems to be the best opportunity to do so.

One way of seeing exactly when objects are batched together is to make a shader which modifies color (or vertex position) based on the local position of the vertex. Because the object center for batched objects is always the world's zero vector, your color or motion will become very different than expected/intended when the objects are batched together.

That said, this variety in how objects do wind up batching together is why you see variable results. If everything batches together, you'll have two draw calls. Otherwise, 40 of the cars will use up to 80 draw calls.