Hi there
An answer to the specific question you have asked is ...
Declare a variable and initialise it to 0:
unsigned long secondsCounter = 0;
At the end of your loop, add 20 and print it:
secondsCounter += 20;
Serial.println(secondsCounter);
BUT ...
How do you know your loop will take exactly 20 seconds to run? It would be better to use the millis() function to check how much time has passed since the last time round the loop and print that.
For example:
unsigned long lastMillis = 0;
unsigned long nowMillis = 0;
nowMillis = millis();
Serial.println((nowMillis - lastMillis) / 1000.0, 1); // Forced to float to display decimal part of time, and set to 1 decimal place
lastMillis = nowMillis;
Regards
Ray