Code

#include <LiquidCrystal.h>

//for LCD use
#define RS 12
#define E 11
#define DB4 5
#define DB5 4
#define DB6 3
#define DB7 2

#define PIR_IN 6 //input sensor (bottom)
#define PIR_OU 9 //output sensor (top)
#define BUZ 10 //piezo

#define MAXP 25 //maximum allowed inside
int tot=0; //holds the number of clients, de facto
int res=0; //calling the sensor functions
int red=8; 
int green=7;

LiquidCrystal lcd(RS, E, DB4, DB5, DB6, DB7);

void setup(){
    lcd.begin(16, 2);
  	pinMode(PIR_IN, INPUT);
  	pinMode(PIR_OU, INPUT);
  	pinMode(red, OUTPUT);
  	pinMode(green, OUTPUT);
  	led_green();
  	pinMode(BUZ, OUTPUT);
  
    lcd.print("MAX PERMITIDO:");
  	lcd.setCursor(14,0);
  	lcd.print(MAXP);
 	lcd.setCursor(6,1);
  	lcd.print(tot);
}

void loop(){
  	lcd.setCursor(0,1);
    lcd.print("ATUAL:");
    res = increase() - decrease();
    delay(100);
	if (digitalRead(PIR_IN)==1 && tot>MAXP){ //someone got in even being crowded
      do{ //the piezo it's activated
        tone(BUZ, 200, 500);
        delay(350);
        tone(BUZ, 400, 500);
        delay(350);
        tone(BUZ, 200, 500);
        delay(350);
        delay(1000);
      }while(digitalRead(PIR_OU)==0); //until someone leaves
  }
}

void led_green(){
	digitalWrite(green, HIGH);
    delay(100);
  	digitalWrite(red, LOW);
    delay(100);
}

void led_red(){
	digitalWrite(green, LOW);
    delay(100);
  	digitalWrite(red, HIGH);
    delay(100);
}

int increase(){ //input sensor function
    if(digitalRead(PIR_IN)){
		tot++;
      	lcd.print(tot);
      	delay(700);
    }if (tot>=MAXP){ //turn red if the maximum has been reached
    	led_red();
    }else{
      	led_green();
    }
  	return tot;
}

int decrease(){ //output sensor function
    if(digitalRead(PIR_OU)==1 && tot>0){
		tot--;
      	lcd.print(tot);
      	lcd.print(" ");
      	delay(700);
    }
  	return tot;
}