Like you count anything, really: By declaring a variable and increasing it, once a certain condition is met.
In this case, that condition would be contact with the wall, which you can detect by using the CharacterBody2D’s function is_on_wall(). Just make sure that you call it after calling move_and_slide and that you invert the direction right away (so the boss will immediately move away from the wall next frame).
You can use direction_to() to figure out in which direction to move, and distance_to() will tell you how far you have to move:
var direction : Vector2 = global_position.direction_to(target_position)
var distance : float = global_position.distance_to(target_position)
Then just modify your velocity vector accordingly.