when we add dynamically in list item..so we need to ADD_LIST_ELEMENT to add elements in list..There are different ways of populating the list item. Using ADD_LIST_ELEMENT is one of the method. However, the preferred method is to create a record group and populate the list using the record group using populate_group and populate_list (online help would certainly have some examples for this).
which trigger we write this ??Depends on the requirement. If you want the list to be populated when the form is loaded, you would be using when-new-form-instance trigger. However, if you want to filter the list based on some condition, you would write it in the WHEN-VALIDATE-ITEM trigger of the filter field / when-button-pressed trigger of the button which filters.
time how much values we can add ??I don't understand this. Is your question is "at a time how many values can be added"? then, there is no limit as such. Or, if your question is, "how much time it takes to add a value", it all depends on many factors like, network etc.