// removing by value
ListItem itemToRemove = DDL.Items.FindByValue("value"); 
if (itemToRemove != null) 
{ 
    DDL.Remove(itemToRemove); 
}
// removing by index number
DDL.Items.RemoveAt(index_number);
 

int count = DDL.Items.Count - 1; 

for (int i = count; i > 0; i--)
{
    if(i > 5)
    {
        DDL.Items.RemoveAt(i);
    }
} 

for(int i=5;i<lstItem.Count;i++)
{
 lstItem.RemoveAt(i);
}

List<int> list = new List<int>();

foreach (int i in list)
{
list.Remove(i);
}