You can connect up to 7 stepper drivers to the expansion connectors of the Duet. The outputs are 3.3V with limited current capacity, so we recommend you use this https://www.duet3d.com/DuetAddons/Expansion_Breakout to boost the signals to 5 external large stepper drivers.
You can use the M584 command in the boot-time configuration file to remap axes to your external drivers, and reallocate the spare internal drivers as additional extruders if you wish.