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).