Is there an available equation to tell the optimum mesh density, "element number "
 

In ABAQUS the higher element number I select, more exact solution I get. However that will take longer time on PC for complicated geometries.
SO please, Is there any available equation I could use to determine the optimum element number !? is it different than the mesh density definition !!

Would you support me with some references ...
Thank you in advance
Kind regards