An Amiga can have as many hard drives as you can connect physically. A SCSI controller can control up to 7 devices, with multiple controllers you can have even more.
The firmware of the harddrive controller should mount existing partitions on the harddrive, provided that is autoboot capable. Each partition has a flag which sets it as bootable or not. Each bootable partition has a boot priority. The partition in the system which is bootable and has the highest boot priority will boot the computer.
If the firmware of the controller is RDB-compatible, you can change partitions, flags and priorities with HDToolbox, HDInstTool or every similar program.
If the firmware is not RDB-compatible you must use the software which came with the controller to change partitions.
Note that the harddrive as such will show nowhere except in the partitioning program (HDToolbox etc.). Only partitions show in AmigaDOS.