The more you stop down (use a smaller aperture) the lesser the effect of lens aberrations on sharpness. However the more you stop down the greater the extent to which diffraction reduces sharpness.
So there is a crossover where reduction in aberrations is cancelled out by increased diffraction. This is usually about f/8, though for really good lenses (with fewer aberrations to start with) it can be f/5.6 or even f/4.
A secondary factor is depth of field. The more you stop down the more of your scene will be in focus, improving the apparent sharpness as less of your objects will be blurred. Again you get a crossover point where the gains from stopping down are cancelled by the increased diffraction. The crossover point depends on your subject (how flat it is) and shooting distance. It may be the best average sharpness is obtained at f/16 or f/22