There are 16 trump cards in JJDD — 14 spades plus the two Jokers, with the 2♦ making it feel like 15 (the 2♠ is one of the 14 spades, and the 2♦ counts as trump for cutting purposes). Knowing how many are left in play at any moment is the difference between guessing and knowing whether your King of Spades is safe to lead.
The starting count
Sixteen trump-tier cards in the deck: Big Joker, Little Joker, 2♦, then A-K-Q-J-10-9-8-7-6-5-4-3-2 of Spades. Subtract your own trump from that number and you know how many are in the other three hands combined.
If you have 5 trumps, the other three players have 11 between them — roughly 3.7 each. If you have 2, they average 4.7 each. Your bidding and playing decisions should follow from this basic math.
Tracking as you play
Every time a trump is played, subtract one from the number of trumps still in play. It sounds obvious. In practice, players lose count during the second hand and stop tracking.
The trick is to only count the top-tier ones you actually care about: Jokers, Power 2s, Ace, King. Track those four categories and you don't need to remember every 5 of Spades that's been dropped.
What to do with the number
If the two Jokers, both Power 2s, and the Ace of Spades are all gone, your King of Spades is now the highest card in the game. Lead it with confidence.
If none of them are out yet, your Ace is not safe. Play the King only if you're forced. Save your trumps until the top of the deck has been drained.
The number also tells you when spades are 'exhausted' — when the number left in other hands drops to 1 or 0, you can lead non-spades knowing nobody can cut you.
