How to add user controls(.ascx) inside repeater on button click

I have a repeater, in which i placed a linkbutton and a placeholder. what i want is when user clicks on linkbutton, user control(that i created in UserControls folder) get added in placeholder as many times as button is clicked by user in current repeater item.
Till now i'm able to add multiple user controls on linkbutton click. but when i click some other button or link, page is posted back and all those dynamic user controls just disappear.

Next, when i again click linkbutton, number of user control before post back + 1 user controls loaded again.

following is my code below :
// page load
protected void Page_Load(object sender, EventArgs e)
{
if (Session["TherapistName"] == null)
{
Response.Redirect("~/Logon.aspx");
}
if (!IsPostBack)
{

FillRepeaterInterventions();
ArrayList ar = new ArrayList();
ViewState["array"] = ar;
foreach (RepeaterItem item in rptInterventions.Items)
{
int itemIndex = item.ItemIndex;
PlaceHolder placeholder = (PlaceHolder)item.FindControl("Placeholder1");
LoadUserControl(placeholder, itemIndex);
}
}
}
//protected void rptInterventions_ItemCommand(object source, RepeaterCommandEventArgs e)
{
if (e.CommandName == "AddIntervention")
{
int itemIndex = e.Item.ItemIndex;
PlaceHolder placeholder1 = e.Item.FindControl("PlaceHolder1") as PlaceHolder;
int count = ((ArrayList)ViewState["array"]).Count;
((ArrayList)ViewState["array"]).Add(count + 1);
LoadUserControl(placeholder1, itemIndex);
}
}

// load user control method

protected void LoadUserControl(PlaceHolder placeholder1, int itemIndex)
{

placeholder1.Controls.Clear();
if (((ArrayList)ViewState["array"]).Count > 0)
{
for (int i = 1; i <=((ArrayList)ViewState["array"]).Count; i++)
{
UserControls_ucNewIntervention ucNewIntervention = LoadControl("~/UserControls/ucNewIntervention.ascx") as UserControls_ucNewIntervention;
ucNewIntervention.ID = "webUC" + itemIndex + i.ToString();
placeholder1.Controls.Add(ucNewIntervention);
}
}
}