Depends on what kind of advertising you are selling. If you are selling backlinks you dont even need to have traffic if you have a good
PR on your site. If you are selling banner ads, so i think there is no such thing as the minimum views for selling it - some advertisers ask for minimum website traffic, others - not. But if you are selling ads on website, that doesnt receive any visitors you wont make money and you wond find advertisers. I think you should have at least 100 unique visitors daily to find some advertisers.