I am afraid you don't understand.

Device mapper is a kernel subsystem that allows creating virtual block devices and mapping them to real devices.

You can for example create a device called /dev/dm9 which is a device of size 4GB starting at 24GB of /dev/mmcblk0. This way, even if you don't have an mmc block device for this partition, you can still use it.

kpartx is a tool that automates this. You point it to a block device, and it creates a dm device for each partition on the device, regardless of partition number limitations of the underlying block device driver.