Normally Linux kernel had support 8 loop ( loop0 to loop7 )device compiled into it. If you have tried to configure virtualization system asking more loops to run in the server, we can build that loops in CentOS-6/RHEL-6 and CentOS-5/RHEL-5,

RHEL 5 / CentOS 5:

Open your /etc/modprob.conf file and add the below line. 

options loop max_loop=64

# vim /etc/modprobe.conf

alias scsi_hostadapter megaraid_sas
alias scsi_hostadapter1 ahci
alias net-pf-10 off
alias ipv6 off
options ipv6 disable=1
alias eth0 tg3
alias eth1 tg3
options loop max_loop=64

Once saved the modprob.conf file :wq! execute below command and reboot the system.

# rmmod loop
# modprobe loop

There are 64 loops will be created in /dev (loop0 - loop63) also check with below command,

# ls -la /dev/loop*

brw-rw---- 1 root disk  7,   0 Jan  5 15:19 /dev/loop0
brw-rw---- 1 root disk  7,   1 Jan  5 15:19 /dev/loop1
brw-rw---- 1 root disk  7,   1 Jan  5 15:19 /dev/loop2
brw-rw---- 1 root disk  7,   1 Jan  5 15:19 /dev/loop63

RHEL 6 / CentOS 6

Do you need to add loop device in Redhat and CentOS 6  permanently. There are two options available, one is configure in grub.conf file and another one is /etc/rc.local

Option: 1

Open the grub.conf file and add a line (max_loop=64),

# vim /etc/grub.conf

title CentOS (3.14.13-1.el6xen.x86_64)
        root (hd0,0)
        kernel /vmlinuz-3.14.13-1.el6xen.x86_64 ro root=UUID=sk87dsed-naags-4972-bb4d-a4jg79gj34c rd_NO_LUKS rd_NO_LVM LANG=en_US.UTF-8 rd_NO_MD SYSFONT=latarcyrheb-sun16  KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet max_loop=64

Option : 2

Open your /etc/rc.local file and add below line,

# vim /etc/rc.local

/sbin/MAKEDEV -m 64 /dev/loop

Finally, restart your system,
# init 6

Do you want to manually build loop devices, use the following command to create 256 loop devices.
# /sib/MAKEDEV -v /dev/loop