How to increase the amount of probing points for a BLTouch sensor in Marlin firmware?
You need to change the constant value(s) in your Configuration.h file of your Marlin version from:


// Set the number of grid points per dimension.
  #define GRID_MAX_POINTS_X 3
  #define GRID_MAX_POINTS_Y GRID_MAX_POINTS_X


to:


// Set the number of grid points per dimension.
  #define GRID_MAX_POINTS_X 5
  #define GRID_MAX_POINTS_Y GRID_MAX_POINTS_X


This shows that to go from a 3x3 grid to a 5x5 grid you only need to change one value. Do note that you can set GRID_MAX_POINTS_Y to any value other than GRID_MAX_POINTS_X; replacing GRID_MAX_POINTS_X in the line with GRID_MAX_POINTS_Y to 4 would yield a 5x4 probe grid:


// Set the number of grid points per dimension.
  #define GRID_MAX_POINTS_X 5
  #define GRID_MAX_POINTS_Y 4