In reply to Ope Coker.

I don't think there is a limit of 15. I think the labels are overlapping and the default FITPOLICY= is to THIN the labels that would otherwise collide.
Try the following:
1. Make the graph wider so that there is more space to display the labels. For example, use
ODS GRAPHICS / WIDTH=1000px HEIGHT=400px;
2. USE the FITPOLICY=STAGGER on the XAXIS statement and see if that enables the tick marks to display without overlapping.