You count turns for “Final Countdown” like this: Turn 1 is the turn you activate it, Turn 2 is your opponent’s turn, Turn 3 is your turn after you activated “Final Countdown”, etc.
“Final Countdown” is immediately sent to the Graveyard when it resolves.
You can activate “Pyro Clock of Destiny” to advance the turn count by 1 for “Final Countdown”. “Final Countdown” does not have to be on the field when you use “Pyro Clock of Destiny”.
You win when the 20th turn ends, before the Draw Phase of the 21st turn.
When your opponent chains “Imperial Order” to your “Final Countdown”, the effect of “Final Countdown” is negated and you cannot start counting turns. But if “Final Countdown” is activated and resolves, and later “Imperial Order” is activated, it will have no effect on “Final Countdown” because “Final Countdown” will be in the Graveyard.
When you activate “Final Countdown”, and your opponent activates “Final Countdown” during his next turn, and then uses “Pyro Clock of Destiny” to advance his “Final Countdown” by 1, both “Final Countdowns” win at the same time and the Duel is a DRAW.
Netrep Rulings:
If “Last Turn” is activated on Turn 20 of “Final Countdown,” “Last Turn’s” effect will take Priority.
Netrep Q&As:
Q: Would activating a second “Final Countdown” stop the countdown of the first one or would two separate counts be kept?
A: Two separate counts would be kept. Curtis Schultz, 01/20/2004