The limit is for 20 million polygons per subtool. This is actually a pretty dense mesh but you aren’t stopped from sculpting if you reach the limit, you just can’t sub-divide that subtool again to add more polygons. This might restrict the amount of detail you can add. Also, it will depend on your mesh when you reach the limit because the number of polygons is multiplied by four each time (each quad is divided into four new quads). So for example, if you have a mesh with 8 million polygons, dividing again would give you 32 million polygons, so you won’t be able to do that. But you could spilt the model into two pieces (subtools) which, if they were 4 million polygons each you would then be able to divide so that they were 16 million polygons each.
And yes, ZBrush & ZBrushCore are brilliant with a tablet. Even a cheap tablet is better than a mouse but make sure that it is a Wacom or Wacom-compatible tablet.