how do i count leftover enemies in screen?

hy guys, i'm confused how to count leftover enemies in screen (they are not destroy), let'me to show my code

public int ScoreValue; public int penalty;

// Increment score by ScoreValue if enemies destroy

if (health > 0) { health--; }

         //Check the Health if less or equal 0
         else if (health <= 0)
         {
             Instantiate(Explosion, transform.position, transform.rotation);             
             SharedValues_Script.score += ScoreValue;                                    
             Destroy(gameObject);                                                        
         }

so, i want count leftover enemies are not destroy in screen.

if (health == 1) { SharedValues_Script.score -= penalty; }

but, i dont no, where i must put it. because i try put like this,

if (health == 1) { SharedValues_Script.score -= penalty; }

       if (health > 0)
         {
             health--; //Decrement Health by 1
         }

         else if (health <= 0)
         {
             Instantiate(Explosion, transform.position, transform.rotation); 
             SharedValues_Script.score += ScoreValue;              
             Destroy(gameObject);    
         }

the output is: if player attack and enemy health is decrease, then penalty is increase 1(+=1), not leftover enemies counting..

Wish there That will help, thank you.. ^^