v_mart should not be static and should be a local variable inside page_load method
Make below change in Page_load
int v_Mycart =0;
if (!IsPostBack)
{
if(Session["mycart"] != null)
{
v_MyCart = (int)Session["mycart"];
}
LblMyCart.Text = Convert.ToString(v_MyCart);
LblMyCart.Visible = true;
}