Hypex at your service
How many Fusion Amp slave units can I connect to the Fusion Amp master unit?
A maximum of three slave units can be connected to one master unit.
https://www.hypex.nl/FAQ-Hypex/Fusion-Amp-Family/How-many-Fusion-Amp-slave-units-can-I-connect-to-the-Fusion-Amp-master-unit