I try to factorized like this
List<Task<string>> Tasks = new List<Task<string>>();
foreach( DataRow item in dt.Rows)
{
Tasks.Add( RestApIHelper.GetByLastDate(item["Url"].ToString(), fetchAllFirstTime.token, Convert.ToDateTime(item["LastDate"])) );
}
var responses = await Task.WhenAll(Tasks);
foreach(string response in responses)
{
var data = JsonConvert.DeserializeObject<List<createReservation>>(response);
roomCreateOrUpdateViaAPI(data, "insert");
}