How many hard drives are in this Mac mini?
You have one drive.
Your diskutil list command tells you everything you need to know:  there's only one entry that has a &quot;physical&quot; characteristic to it and it's disk0.
When you boot into Recovery, there are many RAM disks that get created and in your case they are disk1 through disk19.  If you notice on each identifier, it describes it as a &quot;disk image&quot; as in the example below:
/dev/disk1 (disk image):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        +2.1 GB     disk1
   1:                  Apple_HFS OS X Base System        2.0 GB     disk1s1

What you want to look for is anything that says &quot;internal, physical&quot; because if the seller assured you &quot;there are two hard drives,&quot; you would have (in a Mac mini) two identifier with &quot;internal, physical&quot; descriptors like that of disk0:
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            999.3 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3

What the seller showed you was a (Bootcamp) partition.  When booted, the partitions appear to the OS as their own separate drives.