You may place as many banner modules as you like on a page, so you could just have one group called MY AD (ex) and assign this to as many banner modules as needed across many pages.
To get further control over what ads appear on each page you will need a little more trickery, as an example start with a group name for example
MYBLOCK as you your assigned group name and lets say MYBLOCK represents a 200 x 200 image , text , video etc
Then you wanted to add the abilty to have this show on what ever page you wanted at will, then create a system to do this by first looking at your page structure, so say i had pages like this
home
home > my first page
home > my second page
home > my third page
home > my third page > third page support
home > my fourth page and so on
Then when making a banner you yould name the group as an example
HOME/THIRDPAGE/THIRDPAGESUPPORT/MYBLOCK
Once you have done that copy the banner ad ( you will see copy at the bottom ) then simply rename / repeat until back at home this would leave your copies looking like this
HOME/THIRDPAGE/THIRDPAGESUPPORT/MYBLOCK
HOME/THIRDPAGE/MYBLOCK
HOME/MYBLOCK
if you wanted to take it further you could then copy and rename the master ( first file) rename it and follow down again as
HOME/secondPAGE/MYBLOCK
HOME/firstPAGE/MYBLOCK
with this you then add the banner module to each page and assign the name as needed, so on my third page support i would name the group
HOME/THIRDPAGE/THIRDPAGESUPPORT/MYBLOCK
from here once the structure is in place you can add as many ads to that group as you like and they will auto fall into the correct place ( use the item name to identify different ads )
Then if you wanted to add different sizes say a tower then create a new last extension say
HOME/THIRDPAGE/THIRDPAGESUPPORT/MYTOWER and away you go again.
You can get an idea of how it all works HERE all levels are target as to thier needs and it totally runns on banners set up as above