The number of domain controllers that can be deployed on an Active Directory infrastructure depends on the budget and available resources (computer resources, space, power, and connectivity). Based on the number of domain controllers, engineers will need to decide to either run all FSMO roles together or distribute them among domains controllers. It is recommended that you have at least two domain controllers in a site. One will be holding the FSMO roles, and other one will be kept as a standby domain controller. In the event of a primary server failure, FSMO roles can be hosted on standby domain controller.