Add to cart only create i column

when i add to cart i using Session["Cart"] = new List<string>() { Id }; which is get the id pass from the query string but when i preview on the cartview event i add 2/3 product, it only will show 1 column, which is the lastest. why it will replace? Based on the example when add a product it need a foreach, do i also need it? my cart will alway appear on 1 column  thx for help ~ 
protected void btnAddtoCart_Click(object sender, EventArgs e) { Session["Cart"] = new List<string>() { Id }; }