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From this answer the solution is to
modprobe loop max_loop=64
Which makes me allowed to use 64 loopback devices then
mknod -m 660 /dev/loop8 b 7 8
To create the devices. I did this for 8, 9, 10 and 8,9 works but 10 does not.
I then tried loopa to loopf and tried to mount a 11th device and i get the error
Error: Failed to set up a loop device:
How do I make >10 loop devices?
Make sure you are running mknod -m 660 /dev/loop10 b 7 10. The format is mknod -m 660 /dev/loop<ID> b 7 <ID> where ID is the same.
Update [07/10/2014]
I also found a good blog post to always have more at boot. See https://yeri.be/xen-failed-to-find-an-unused-loop-device
Update [05/25/2016]
I run a CentOS server, and I found that this post was also helpful when the other methods don't work.
This makes my new favorite method:
MAKEDEV /dev/loop
It creates 256 loop devices (which is the max without modifying the kernel).
If you want to find a free major loop device number amidst non-numbered loop device names(e.g. /dev/loop-something), you can use this command to find the biggest one and add 1:
find /dev -type b -print0 | xargs -0L1 bash -c 'stat -c "%t %T" "$1" | awk "/^7 / { print(\$2); }"' '' | sort -nru | head -n1
mknod -m 660 /dev/loop10 b 7 10. It worked fine for me. What distribution are you using?losetupormount -o loopon a loop device that's already in use (even if you reach that device through a different name).