First of all, you’ll need a variable (possibly a float or an int) in a script that stores your score. Adding points when you hit the plane is easy: use OnCollisionEnter or OnTriggerEnter (depending on your colliders) on a script in a plane to determine if it was hit, and just add the appropriate amount of points to the score-variable.
Subtracting points when you miss could be a bit trickier. I suppose the easiest with a 2D invaders-style game would be to have an invisible plane at the top of the screen, which, if hit, means a plane was missed and a point can be subtracted.
In a 3D game you could maybe have a timer on the bullet. Have the time be long enough so that no plane can be hit after that. You could also have invisible planes surrounding the playing area, which, if hit mean the same thing as in the 2D version.