I have 21 disks after running “diskutil list”

Why do you need to do this? Is the drive not working as is? Those items in the second pic appear normally. They do not require reformatting the drive. If you need to reformat the drive then do so using Disk Utility. Less can go wrong. Do you want to change from APFS to HFS+? If so, then do this:

Install El Capitan or Later from Scratch

If possible backup your files.

1. Restart the computer. Immediately at the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Install OS X and click on the Continue button.

This will install the version of OS X you had installed.

Yes. You do have 21 snapshots that are kept in your VM volume. APFS creates four volumes:

/dev/disk8 (synthesized):

#: TYPE NAME SIZE IDENTIFIER

0: APFS Container Scheme - +199.6 GB disk8

Physical Store disk5s6

1: APFS Volume High Sierra Recovery 22.0 GB disk8s1

2: APFS Volume Preboot 25.2 MB disk8s2

3: APFS Volume Recovery 520.8 MB disk8s3

4: APFS Volume VM 2.1 GB disk8s4

The above is from my startup volume which happens not to be disk0 but disk8 on my computer. This is one partition of an external 1TB SSD that happens to be used for startup. The disk8s0 is the storage space of the container. The VM volume is 2.1GBs in which there are various of these snapshots. There is a 22GB partition that contains a bootable volume. This is not the same as the Recovery HD volume that is just under 521MBs. And, lastly, is the 25MB Preboot volume. This is the typical arrangement for an APFS volume. The actual physical store consists of slices 1 to 4. These are part of the larger SSD that has been "partitioned" into two volumes. One is an APFS volume as shown above. The other is an HFS+ volume on the same SSD, disk5. It appears as any normal HFS+ volume would appear except that it also contains the Physical Store of the APFS volume on disk5s6.

I hope I haven't made this too complicated. I'm no expert at this so I am relating what I have learned from others.

That is merely an aspect of using AFPS. Those are virtual disk known as snapshots.

These snapshots may be caused by Time Machine to enable Time Machine to work with an APFS drive. I'd love to explain it more but I don't really understand it all. See the Snapshots section of this article where it is far better explained than I can do: Taking Stock- Using APFS in High Sierra 10.13.1.

Gotcha!