Count how many items are touching to a certain object

You can use OnCollisionEnter, and a public integer variable attached to the floor object, for example:

 GameObject floor;
 
 void Update () {
 floor = gameObject.Find("Floor");
 }
 
 void OnCollisionEnter2D (Collision2D col) {
 if (col.name == "Floor"){
 floor.GetComponent<BallCount>().balls ++;
 }
 }
 
 void OnCollisionExit2D (Collision2D col){
 if (col.name == "Floor"){
 floor.GetComponent<BallCount>().balls --;
 }
 }

Then, add a script to the floor called BallCount:

 int balls;

That's it really. All the script attached to the floor needs to have is that integer value that stores the number of balls. I don't use 2D often, so I may have typed an error in the collider methods.