I would like to know how much unique visitors I have on some webapplication. In this webapplication are a lot of site collections and pages. So everything needs to be included. I would like also to know it for 1 month. What is the best practice to do that?
Go to Site Actions -> Site Settings
Click "Site Web Analytics reports" under Site Actions
Click "Number of Daily Unique Visitors"
This will give you the daily break down and total for 30 days for the web application.
You can change the time range by clicking "Change Settings"