In fact this has happened in this site. But this rare, rather than the norm. Most sites only display one banner, usually at the top of the page. Some sites will display 2 banners in each page, one at the top and the other at the bottom. Some sites display 2 different banners on each page, while others display the same banner at the top and bottom of each page.
Some sites display banners on a timed rotation system.
The interval between each banner display can be as short as 30 seconds or as long as 5 minutes. Each display of a new banner counts as one impression, no matter how it is displayed. It is always best to go for the top fold of page, as the viewer is more likely to notice your banner, while the page is downloading. You should expect to pay a lot less for bottom fold of page banner spaces. Think how many times you have visited a page, scrolled half way down and clicked on a link.