If you are new to the RadDataPager control, you can consider reading the Getting Started topic before continuing.
The RadDataPager splits the data into separate pages with a certain size. In order to specify the size of the page you have to use the PageSize property. By default it's set to 0 and all the data gets displayed in one and only page. Here is an example of a RadDataPager with a page size of 5 and 20 items.
<telerik:RadDataPager x:Name="radDataPager" PageSize="5" />