There are three main tile types on Windows Phone 8. Two of them use the little number in a circle to show a count. The one that you're likely seeing is what's called a "flip" tile on Windows Phone 8. (It was the standard tile type for WP7.)
The flip tile type can show a background image, a title at the lower left, and a optional count at the upper right. It also supports a "back" that can show another image, another title, and some text along the top (but no count). If a "flip" type has both sides defined, the tile animation shows it flipping over several times when you view the Start screen.
There's also a "cycle" tile that can show up to ten images one after the other. Like the flip tile, it has a title at the lower left and an optional count at the upper right. The final common tile type is the "iconic" tile, which is how the email, Store, and many other apps work. The iconic tile shows a white icon for the app and an optional large-sized count next to the icon.
You can learn more about them and see more examples of the three types in the small, medium, and wide sizes at
Live Tile Templates in Windows Phone 8 - Nokia Developer Wiki.