On guided tours, the guide will make the necessary repairs.
On self-guided tours, you should be prepared to make basic repairs like fixing flats. If there's a difficult repair, you may have to call the tour operator's emergency service number or go to a repair shop in the nearest town. Let the tour operator know, and you'll get reimbursed for repair costs. If necessary, you'll receive a new bike. Since many tours are in the countryside, you may wish to rent a cell phone or get your home cell phone activated for international travel.
More about guided and self-guided tours