So the code looks like this :
for (int dynamicControlCount = 0; dynamicControlCount < 5; dynamicControlCount++)
{
// Rating Control for taking user's rating.
AjaxControlToolkit.Rating rating = new AjaxControlToolkit.Rating();
rating.ID = "rating" + dynamicControlCount;
rating.BehaviorID = "ratingBehaviour" + dynamicControlCount;
rating.StarCssClass = "rating";
rating.WaitingStarCssClass = "waiting";
rating.FilledStarCssClass = "filled";
rating.EmptyStarCssClass = "empty";
// HiddenField to store selected rating.
HiddenField hfRating = new HiddenField();
hfRating.ID = "hfRating" + dynamicControlCount;
hfRating.Value = "0";
// Add the Rating control and HiddenField to a Container(PlaceHolder/Panel).
// CODE
// Attaching Javascript function to store selected rating in a HiddenField on clicking the Rating control.
rating.Attributes.Add("onClick", "storeRating('" + rating.BehaviorID + "', '" + hfRating.ClientID + "');");
}
// Javascript function to store selected rating in a HiddenField.
function storeRating(ratingBehaviourID, hiddenControlID)
{
// Collect selected rating.
var currentRating = $find(ratingBehaviourID).get_Rating();
// Store selected rating value in a HiddenField.
document.getElementById(hiddenControlID).value = currentRating;
return false;
}