count selected gridview rows in javascript

1    protected void Page_Load(object sender, EventArgs e)
2    {
3        var list = (new[] { new { ItemID = 1 }, new { ItemID = 2 }, new { ItemID = 3 }, new { ItemID = 4 }, new { ItemID = 5 }, new { ItemID = 6 }, new { ItemID = 7 }, new { ItemID = 8 } }).ToList();
4        gvPurchaseOrderDetails.DataSource = list;
5        gvPurchaseOrderDetails.DataBind();
6    }