Add a label control to your Repeater's ItemTemplate. Handle OnItemCreated event.
ASPX
<asp:Repeater ID="rptr" runat="server" OnItemCreated="RepeaterItemCreated">
<ItemTemplate>
<div id="width:50%;height:30px;background:#0f0a0f;">
<asp:Label ID="lblSr" runat="server"
style="width:30%;float:left;text-align:right;text-indent:-2px;" />
<span
style="width:65%;float:right;text-align:left;text-indent:-2px;" >
<%# Eval("Item") %>
</span>
</div>
</ItemTemplate>
</asp:Repeater>
Code Behind:
protected void RepeaterItemCreated(object sender, RepeaterItemEventArgs e)
{
Label l = e.Item.FindControl("lblSr") as Label;
if (l != null)
l.Text = e.Item.ItemIndex + 1+"";
}