Why when I select dynamesh resolution at 2048 I get not a 4 mil vertexes mesh but a 100 000 vertexes mesh?
The resolution of the mesh will depend on its size. Basically when you set the Dynamesh resolution to 2048 you are setting the resolution for a cube of one ZBrush unit. If your model is smaller than the cube then it will only use part of that resolution. Increasing the size of your model will increase its resolution.
However, remember that Dynamesh is meant to be used for creating the initial stage of your model - roughing out ideas until you get to a good starting point. It is not a good way to add fine detail. For that, dividing you mesh to create subdivision levels is the best solution.
See here for more about Dynamesh:
http://docs.pixologic.com/user-guide/3d-modeling/modeling-basics/creating-meshes/dynamesh/methods/
Thanks! I use dynamesh for 3d printing, because my mesh contains holes and isn’t solid(for easier sculpting and modeling)