1. if you are using SQL Server, try
select identity(int,1,1) as 'id', * into #mytemp from YourTable
select * from #mytemp
2. you could add a column to the DataTable:
DataTable1.Columns.Add("ID",typeof(int));
int i=0;
foreach (DataRow dr in DataTable1.Rows)
{
dr["ID"] = ++i;
}
3. or you can use in your template
<%# Container.ItemIndex+1 %>