The router by default comes with one power supply. If you need a second one, you can use one 2300 to provide power redundancy.
Have a look at table-2 in the data sheet:
The Cisco 2911, 2921, and 2951 allow for power redundancy through the use of an external RPS device, thereby decreasing network downtime and protecting the network from power-supply failures.
• Redundant power on the Cisco 2900 Series is supported through the Cisco RPS 2300 Redundant Power System. You can use the Cisco RPS 2300 to provide redundant power for Cisco 2900 Series ISRs as well as Cisco Catalyst® switches.
• In order to use the Cisco RPS 2300, an external RPS adapter is required (configurable option) to connect the platform to the external RPS.