I have seen in many sites the feedburner button where it shows the number of readers. How to get that?
It's called FeedCount. The new service that is now open to all publishers is FeedCount. This is a customizable chicklet that shows the current number of subscribers your feed reaches. It's suitable for display on your home page or wherever you'd like to show your current circulation. To generate a FeedCount for your FeedBurner feed, just go to the Publicize... link next to your feed on the "My Feeds" page and customize your FeedCount chicklet. When you are done, copy the HTML snippet FeedBurner generates and paste it into your own page templates.