If you would like to bulk add tags to customers/contracts, you can do so in a few different ways.
Customer Map
If you go to Customers->Customer Map and highlight some customers, you can bulk add and remove tags from the three-dot menu on the right. If the customer has multiple contracts, it will add or remove tags from all of the contracts on the customer's account.
Once you click on "Bulk Customer Contract Update", you will get this pop-up window:
You can both bulk add and bulk remove tags from this window.
Learn what else you can do from the Customer Map here.
Route List
If you go to Routing->Route List, you can search for customer with certain contracts and update contracts individually or in bulk. See how here.
You can bulk update from the list view as shown in the screenshot below.
Or you can update from the map view.