When constructing the scroll wheel, I draw only one ellipse. You certainly can choose to draw two, and depending on your comfort level with visualization and cylinders in general, it may be the better choice. Still, here I made that decision in order to reduce the amount of clutter. As with everything else, with all of the subdivision, clutter is a major factor here. Still, the solidity of your forms and construction is of the highest priority. One of the main reasons I made that decision was because this was intended as a demo, so clutter would have made it much more difficult to follow.
You could also choose to build this cylinder as a box initially, but again, that would further compound the clutter problem and may not be entirely necessary. Make sure you pay close attention to the proportions and sizing of the scroll wheel in your reference image before beginning this phase.