I’ve been using a lot of VMs recently. By default the number of loopback devices set in Ubuntu is just 8. So if you want to mount 10 VM disks, your out of luck.
An easy way I’v found to increase loopback devices is:
edit /etc/modprobe.d/options
and enter the line (replace n, for the number of loopback devices you want. Max. number is 255).
options loop max_loop=n
