To reproduce: use the following code snippet:

 Random rand = new Random();
 List<LineSeries> list = new List<LineSeries>();
 for (int i = 0; i < 15; i++)
 {
     LineSeries ls = new LineSeries();
     ls.LegendTitle = "Series" + i;
     list.Add(ls);
 }
 for (int i = 0; i < 1000; i++)
 {
     foreach (LineSeries s in list)
     {
         s.DataPoints.Add(new CategoricalDataPoint(i, rand.Next(0, rand.Next(5, 20))));
     }
 }
 radChartView1.Series.AddRange(list.ToArray());

 this.radChartView1.ChartElement.LegendPosition = LegendPosition.Bottom;
 this.radChartView1.ChartElement.LegendElement.StackElement.Orientation = Orientation.Horizontal;

Resolution: 
The issue is duplicated with item IMPROVE. RadChartView - ChartLegend should be able to wrap its items. From Q1 2015 when there are many items in the chart legend a scroll bar will appear so users can scroll through the items.
More info in the following feedback item http://feedback.telerik.com/Project/154/Feedback/Details/149848-add-radchartview-when-the-legend-has-many-items-a-scrollbar-should-appear