You can fit 3 Trucks inside your Garage.

When you exceed the number of Trucks in your Garage, new Trucks will be placed in separate Garage, which you can see by tapping the Garage button at the left side of the screen.

This will open your Garage, where you can see your Trucks grouped by 3. Look at your garages by swiping or tapping the arrows. Tap twice to choose a Truck.