How to configure the category gridlines in Numbers?
Both your plots have grids but the bottom one has too many.  You need to change the spacing by setting the number of grid lines.
Double click on the plot and in the right window click on axis at the top of that area. This opens a pane with Value X and Value Y labels near the top of the pane.  Below that you'll see Axis Scale which you can set to linear or logarithic. Right below that the pull down for linear or log there is a box that says Steps with major and minor underneth it.
You can enter a whole number in those two boxes and numbers will draw grid lines close to the number you entered. It tries to find a nice spacing to use. See the image below. At the bottom of the pane there are settings for major and minor grid lines.