For a stair railing, you can specify the baluster family and the number of balusters per tread. 
- In a plan view, select a railing. 
-  On the Properties palette, click  Edit Type. Edit Type.
- In the Type Properties dialog, for Baluster Placement, click Edit. 
- In the Edit Baluster Placement dialog, select Use Baluster Per Tread on Stairs. 
- Specify the number of balusters on each tread. 
- Specify the baluster family for the stairs. 
- Click OK.