Ok, well I presumed that you had some code you wanted to do 21 times and then you wanted it to reset. That is what the 'for' code would be used for.
void loop()
{
for (int i=0; i <= 21; i++){
//insert code here
}
}
The code would then repeat 21 times.
Mike's code would just check is the button presses was 21 and then if so, it would reset it which I suppose was what you asked for!
Go Mike! ;D
Mowcius