Minimum 1 no. CI590 is Must. ABB AC500 Redundancy,called HA, is needed CI590. You can add your IO modules on right of CI590. You can use multiple CI590's if you need to have more IO's.Each CI590 will be having 2 No. of CS31(data line) from both the controllers.
Please do go throgh the structure attached.
Source: Codesys Help
Onboard Ethernet port will be use to connect UDP connection to both PLC in case of using redundancy.
If you have any touch panel or any scada system then you have to use CM577 Ethernet module to both PLC for connectivity.
Local IOs (right side to PLC) should be duplicate in both PLC if you have less IO'S or field network is not required.
CI590 has Primary CS31 bus connection and secondary CS31 bus connection as well.so you can connect both PLC to single CI590 Module. In case if you are using Field network arrangement.
Please go through the documents as Flaviomussolin already attached in his reply.