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?