If your site can lead to the succesful sale of a software company / a jet plane worth millions then one unique visitor is enough.
But even with 1 million unique visitors no sale is made then it is worthless to the advertiser.
The answer to your question lies anywhere in between these two examples:
What kind of site you got ? Are the visitors from any specific geographical location or a specific group ? How many visitors [unique and repeat] it gets, How much decision making capability your visitors have ? What kind of product/service may interest your visitors ? Will they buy immediately or just 'window shop'?..............all these factors are needed to get the right solution for an advertiser.
To start with you can place Google's Adsense ads since there is no specific requirement of traffic to be eligible. If the ads are clicked by your visitors you get paid or if there are no clicks then you dont get paid.
If you really got a high profile site with many visitors then you can approach an advertiser directly [or their agency] and discuss in detail.
Prices can vary on many factors: where your ads are going to be positioned can depend on how much you are willing to pay or who the target is [Like a Mercedes may advertise only in Germany but CocaCola may want to advertise world wide. Mercedes may be willing to pay more per click since its a high cost product but for Coke it may not be very critical]
And there are many different types of ads: text ads, banner ads, popups, exit popups, popunders, etc. Google's text ads is most prefered these days since it does not intrude on the design on the website,