internal class Program
{
    private static void Main(string[] args)
    {
        var dt1 = new dt()
        {
            id = 1,
            ds = "20220101",
            de = "20220202"
        };


        var dt2 = new dt()
        {
            id = 1,
            ds = "20220101",
            de = "20220202"
        };

        var l = new List<dt>();
        l.Add(dt1);
        l.Add(dt2);

        //cnt = 2  ?
        var s = l.GroupBy(dt => dt.id).Select(b => new { Key = b.Key, cnt = b.Select(d => d.ds.Concat(d.de)).Distinct().Count() });


     //cnt = 1
        var s2 = l.GroupBy(dt => dt.id).Select(b => new { Key = b.Key, cnt = b.Select(d => d.ds + d.de).Distinct().Count() }); 
    }
}

class dt
{
    public int id;
    public string ds;
    public string de;
}