Disable item after 10 shots.

You need to become more comfortable with your use of if statements.

As it stands, here's what's happening:

You click, your ammo is set to 10. But that's all your if statement is doing. Then, every frame, you subtract 1 from your current ammo (so, either 10 becomes 9 or 0 becomes -1), then you set it straight to 0.

You're already setting your ammo count to 10 in the editor. When you click/fire, reduce your current count by 1 (i.e. "countAmmo -= 1" or "countAmmo = (countAmmo - 1)" as you already did), then check whether you have any ammo left.

 ...
 
 if(Input.GetButtonDown("Fire1"))
 {
      if(countAmmo > 0)
      {
           // Instantiate a projectile or whatever the means is to fire.
           countAmmo -= 1;
      }
      else // if(countAmmo <= 0) -- Commented out because it's unnecessary, but there to show intent
      {
           // You have no ammo!
      }
 }
 
 ...

The important part is in here, now u must adapt it

if(Input.GetButtonDown("Fire1")){//checks for input

   countAmmo--;//count ammo - 1
   if(countAmmo <= 0){//if cound ammo is less or iqual to 0
      myObject.SetActive(false);//turn off object
   }

ps: You should pratice a little more your programing skills