Install any text editor like mouspad, gedit, leafpad (If already installed then ignore this step)
 

Open Terminal and apply the any one command from the following with your favorite editor (Open Applications > System Tools > Terminal or  ctrl+alt+t):-

sudo apt-get install mousepad
                    (or)
sudo apt-get install leafpad
                    (or)
sudo apt-get install gedit        

To open the ports configuration file in the gedit editor , execute the below command.

sudo gedit /etc/apache2/ports.conf

# If you just change the port or add more ports here you will likely also
# have to change the VirtualHost statement in
# /etc/apache2/sites-enabled/000-default.conf
Listen 80
Listen 8080
 
<IfModule ssl_module>
         Listen 443
</IfModule>
 
<IfModule mod_gnutls.c>
         Listen 443
</IfModule>
#vim: syntax=apache ts=4 sr noet

Add more different port numbers for new Koha instance as per your requirement 

(here we are adding two  ports Listen 8000 and Listen 8001 below the port Listen 80 and Listen 8080 in the configuration file.)

# If you just change the port or add more ports here  you will likely also
# have to change the VirtualHost statement in
# /etc/apache2/sites-enabled/000-default.conf
Listen 80
Listen 8080
Listen 8000
Listen 8001
<IfModule ssl_module>
         Listen 443
</IfModule>
 
<IfModule mod_gnutls.c>
         Listen 443
</IfModule>
#vim: syntax=apache ts=4 sr noet