An ISE VM in Azure uses the same license model as one in your own data center. The VM(s) each require a license for the VM itself. The deployment needs licenses for the respective tier (Essentials / Advantage / Premier) and features (TACACS / Device Admin and IPsec) that you require.
ISE failover can be setup but requires at least 3 VMs. If you are asking about having PSN high availability then yes that is setup from the network device side by pointing to both of your nodes as AAA servers.
For ISE in Azure, please see this document for more details: https://www.cisco.com/c/en/us/td/docs/security/ise/ISE_on_Cloud/b_ISEonCloud/m_ISEonCloudOverview.html