Comunicação serial

Projects General Guidance
11 
int numero;
char letras;

void setup() {
Serial.begin (4800);
Serial1.begin (9600);
}

void loop() {

Serial1.print ("?V913");
Serial1.print('\r');
Serial.println("I sent the command ?V913");
delay(10);

   if (Serial1.available() != 0) {
     letras = Serial1.read();
     // Serial.print(letras);
     Serial.print("I read the character ; ");
     Serial.println(letras);
     delay(100);
   }
}

Try the above.
The function 'loop' gets called again and again.
Each time round the loop your are sending the query "?V913\r" - the '\r' is the Carriage Return
However you are only reading one character of any reply from the TIC.