The chart shouldn't normally have any trouble displaying a large amount of data. Perhaps something is preventing it from doing so. Here's a few things you can check:

if you have more than one series, make sure that each series has an equal amount of points. Otherwise, you have to use a composite chart.

Make sure that you're not setting a range on the X axis. Check chart.Axis.X.RangeMin/RangeMax/RangeType properties

As a sanity check try this code and see if it works:

DataTable dt = new DataTable();
dt.Columns.Add("date", typeof(DateTime));
dt.Columns.Add("value", typeof(double));
dt.Columns.Add("label", typeof(string));

for (int i = 0; i < 50; i++)
{
   dt.Rows.Add(DateTime.Now.AddDays(i), i, "label " + i);
}

NumericTimeSeries series = new NumericTimeSeries();
series.Data.DataSource = dt;
series.Data.TimeValueColumn = "date";
series.Data.LabelColumn = "label";
series.Data.ValueColumn = "value";
series.DataBind();

ultraChart1.ChartType = ChartType.LineChart;
ultraChart1.LineChart.TreatDateTimeAsString = false;
ultraChart1.Series.Add(series);