forgive me i am just little confused between two cases

can you explain it for me

because my knowledge is that each db must has it is own instance . So how one instance refer to multiple data bases

if you have no time to explain send me any documentation

Sorry for bother

i am trying to install odoo as a SAAS using port mapping
all i want to do install odoo and using virtual hosting (i will give each client port number) and according to the port number it will be directed to his own database

Do you have any particular reason why you want it to deploy this way? Another option is one Odoo instance and multiple databases using dbfilter (most popular). Do you want to have one Postgres instance and multiple databases or multiple Postgres instances? See https://www.odoo.com/documentation/9.0/setup/deploy.html

my question is : how to install more than a copy of odoo on the same server ???
the question in another way : how to install odoo in different directory ????
can it be /var/www/odoo1/ and /var/www/odoo2/     and so on ????

You should install Odoo from source (not package install), see https://www.odoo.com/documentation/9.0/setup/install.html#setup-install-source