okay, a solution that can be translated into working code. First, counting bricks. You need to have a variable to store how many bricks hit so far. Secondly, you need to check if you have reached that limit.

Lets call it:

int brickHits = 0; // default is 0

Every hit will +1 to this variable.

so you need a function like:

void BrickHit()
{
   brickHits++; // ++ is short for +1 

}

After adding +1 then check if brickHits is above your wanted limit (20)

void BrickHit()
{
   brickHits++; // ++ is short for +1 
   if(brickHits>20)
   { 

       // okay above 20, lets do something, 
       // also remember to re-init the variable for the next weapon drop.

       // here you could spawn your weapon drop, play a sound or whatever.
   }


}