A beam element that is assigned a circular cross-section has a mathematically perfect circle when it comes to computing the stress, strain and deformations in the model.  ANSYS Mechanical represents that perfect circular beam cross-section with 8 sides on the display because that is efficient for display purposes. Representing a cylindrical face on the display requires the graphics engine to facet the surface in order to render it to the display and using an actual cylinder would unnecessarily burden the display processor during dynamic view rotations.

The only mesh refinement needed for a beam element is its length.