How to find which images belong to which /dev/loop

You have to create device nodes into /dev with mknod. The device nodes in dev have a type (block, character and so on), a major number and a minor number. You can find out the type and the major number by doing ls -l /dev/loop0:

user@foo:/sys# ls -l /dev/loop0
brw-rw---- 1 root disk 7, 0 Oct  8 08:12 /dev/loop0

This means loop device nodes should have the block type and major number of 7. The minor numbers increment by one for each device node, starting from 0, so loop0 is simply 0 and loop7 is 7.

To create loop8 you run, as root, command mknod -m 0660 /dev/loop8 b 7 8. This will create the device node /dev/loop8 with permissions specified along the -m switch (that's not necessary as you're probably running a desktop system, but it's a good idea not to let everyone read and write your device nodes).

You need to use losetup -P for creating a -Partitioned loop device, or else you need to partition the original loop device then partx -update the kernel's partition table afterward. The /dev/loop0p devices will appear after the kernel recognizes that the partition has actually been partitioned. Probably this is partly what the intent behind the sleep 5 is - but that should almost definitely be a sync - or both - instead.

Anyway, to demonstrate:

sudo sh -s <<\IN
    losetup -D                          
    fallocate "-l$((1024*1024*1024))" loop        
    printf %s\\n n '' '' '' ''  w y | gdisk loop
    sync; losetup -f loop
    lsblk /dev/loop*
IN

So the above sequence first -Detaches all current loop devices (if any) fallocates a 1GB tmp file, writes a GPT partition table to it and does a single partition on it, then syncs the filesystem and assigns it to the -first available loop device before trying to list with lsblk all available loop devices. It prints:

NAME  MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
loop0   7:0    0   1G  0 loop 

But if I alter the losetup line to read:

losetup -fP loop

...it instead prints:

NAME      MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
loop0       7:0    0    1G  0 loop 
├─loop0p1 259:0    0 1023M  0 loop 
└─loop0p2 259:1    0 1007K  0 loop 

...because losetup first scans the backing file for a partition table. As you can see, it's not very good at it - rather than noting the extra 1M (which is almost definitely the last iteration's partition table, actually) as unallocated space it interprets it as a separate partition, but that's probably because I'm writing to a tmpfs (twice in a row) at the top of a 1GB file hole - working w/ actual data will be much more precise (and zeroing the backing file as your script does w/ dd would handle that anyway). In any case - I can freely mkfs.whatever on the real partition there and afterwards mount it. partx -u on the /dev/loop0 would achieve the same results.