The two important numbers that show a site's popularity (and value to advertisers) are the total monthly page-views, and the number of unique visitors.
A typical ratio of these numbers is 5:1 (for example, 10,000 monthly page views correspond to 2,000 unique monthly visitors), but this ratio varies for different types of websites and different population.
A good number that will give you worldwide advertising visibility is 1 Million page views per month. But even with lower numbers, if you can profile your visitors and privide advertisers more focused data (like age, demographics, profession, areas of interest) you have better chances to make more advertising money.
If you want to get these numbers there are several ways:
1. If you have your own web server (either microsoft IIS or any other) these numbers are logged by the server and you can get it in your web server reproting system.
2. If your web site is hosted, your hosting provider can give you the numbers.
3. If your site is popular enough and already has some traffic volumes, there are public services like Alexa.com or ComScore.com which use statistical probes to give pretty accurate estimates of the web site traffic.
