The Shield of Peace grants you protection from enemy attacks. When you start playing for the first time, you will be provided with a Shield of Peace for 7 days and other players will not be able to attack you, allowing you to grow and develop.
You always have options to purchase a Shield of Peace either in your Warehouse's "Battles" section or with packages that are available on your offers screen. You do not have to use a purchased Shield straight away, i.e. you can keep it for later after purchase.
The time of several Shields doesn't sum up.