if (key == '0'){
Serial.println ("Button 0");
if (Number==0)
Number=0;
else
Number = (Number*10) + 0; //Pressed twice
}
if (key == '1'){
Serial.println ("Button 1");
if (Number==0)
Number=1;
else
Number = (Number*10) + 1; //Pressed twice
}
if (key == '2'){
Serial.println ("Button 2");
if (Number==0)
Number=2;
else
Number = (Number*10) + 2; //Pressed twice
}
etc, etc
Do you notice that your code is basically the same for each digit pressed ?
All you need to do is to test that the key pressed is between '0 and '9', subtract '0' from the key value to get an actual number then add it to 10 times the current total.