During the initial enrollment process, you may register as many devices as you wish, including smartphones, cell phones, landline phones, and tablets. If you need to register another device once you have completed the initial enrollment process, you may do so from the Duo self-service portal.
OIT recommends that you register at least two devices, for example both your smartphone and your office phone. In the event that you lose one device, you will still be able to access 2FA protected systems using your secondary device. If you did not register a second device initially, you register a second device directly from the Duo Login screen. Full instructions can be found at guide.duo.com/add-device.