You shouldn't really use millis for tracking a score.. It's a reference to how long your sketch has been running, used for timing stuff.

But if you insist, you should use a variable to store the millis();
that way, you can choose when to stop storing it, and thus getting a single number.

ie:

int startMillis; // Add this to start with a score of 0
int playerScore; // This is where the real score will be stored
void setup()
{
 startMillis = millis();
}
void draw()
{
 if(millis()-startMillis < 5000){ // If the timer has been running for 5 seconds, or whatever you want here to control the score being set
   doSomethingWithScore();
 }else{ // when that is done, print out the score, and exit
   println(playerScore);
   exit();
 }
}
void doSomethingWithScore()
{
 playerScore = millis() - startMillis;
}

I don't have to use millis(), but I found that to be the easiest thing to use, especially since I'm a beginner.

The game is a simple how long can you avoid being hit, and I though using millis() or something of the sort would be the best thing to give a number for score.