Me got the solution we need to add one property MaxDropDownHeight="220" to the autocomplete box so that we can control the height of the Autocomplete itemlist
<input:AutoCompleteBox MaxDropDownHeight="220" Width="125" x:Name="EmployeeList" MinimumPrefixLength="1"
Grid.Column="0" HorizontalAlignment="Stretch" FilterMode="Contains" Margin="2>
<input:AutoCompleteBox.ItemTemplate>
//ur template
</input:AutoCompleteBox.ItemTemplate>
</input:AutoCompleteBox>