How many bingo cards do you need?

Start with players, then add the spares nobody plans for

The working rule is one card per player, plus about 20 per cent. For a class of thirty that is thirty-six; for a party of twelve it is fifteen. The spare margin is not caution for its own sake — it covers the three things that reliably happen. Somebody arrives after the first call and wants to join the next round. Somebody spills a drink on a card. And somebody, usually a child, wants to play two at once, which is the single most common reason a stack runs out.

If you are running several rounds, decide in advance whether players keep their card between rounds or swap. Keeping is simpler and needs no extra printing. Swapping needs a fresh stack per round, because a player who has already seen a card knows which numbers are on it — harmless in a family game, less so if there is a prize. Swapping also doubles as the fix for the complaint that somebody's card is unlucky, which will otherwise be raised roughly once per round for the entire evening.

Two identical cards will ruin a round, quietly

The failure that spoils a game is not running short — it is printing the same card twice. Two players holding identical cards will win simultaneously on every round they both play, every time, and they will keep doing it until somebody works out why. It reads as a rigged game or a broken generator, and by then the prize has usually been handed out.

This matters more than intuition suggests because the number of genuinely distinct cards is smaller than the number of arrangements. For a 5×5 grid drawn from a 75-number pool, the count of possible cards is astronomically large and collisions are effectively impossible. But most classroom bingo is not that game: it is a picture bingo, a vocabulary bingo, a times-table bingo, where the pool might be twenty-four items for a twenty-four-cell card. With a pool the same size as the card, every card holds every item, and the only thing that differs is the arrangement — so two cards can and will coincide.

The rule of thumb worth carrying: the pool should be comfortably larger than the card. A 5×5 card with a free centre needs twenty-four items, and a pool of forty or more gives you room. If your pool is exactly twenty-four, you are not making bingo cards, you are shuffling one card, and you should expect duplicates.

What actually decides how long a round takes

Round length is set by the win condition far more than by the number of players. A single line on a 5×5 card typically falls somewhere around the fifteenth to twentieth call in a 75-ball game — fast, which is why it is the right condition for young children and for the first round of an evening while people are still finding seats. A full card takes most of the pool and can run past sixty calls. Four corners is faster than a line. A blackout with thirty players will produce a winner sooner than a blackout with five, because more cards mean more chances that one of them is close to complete, but the effect is far weaker than switching the win condition.

The practical consequence for printing: if you are running a fixed-length session — a thirty-minute party slot, one lesson period — choose the win condition first and print for the number of rounds that fits, rather than printing a large stack and discovering that two full-card rounds have eaten the whole session.

Printing: what fits, and the sheet people forget

At one card per page, thirty cards is thirty sheets, which is more paper than most people intend to spend. Two cards per page halves it and still leaves each card large enough to mark with a chunky marker; four per page suits counters or dried beans better than pens, because the cells get small enough that a marker tip covers two at once. If the cards will be reused, print on the heaviest stock your printer feeds reliably — around 160 to 200 gsm — and they will survive a term rather than an afternoon.

The sheet people forget is the caller's. Without one you are drawing items from memory or from a bag, and you have no record of what has already been called — so a disputed win cannot be checked, and it will be disputed. A caller's sheet listing every item in the pool, ticked as it goes, turns an argument into a five-second lookup. The bingo card generator prints one alongside the cards, and treats it as part of the game rather than an extra, for the same reason its puzzles ship with answer keys.

The honest limits

None of this is a guarantee against a tie. Two different cards can still complete on the same call, and with thirty players and a single-line condition it happens often enough that you should decide the tiebreak before the game rather than during it — share the prize, or play one more call and take whoever gets a second line. It is a five-second decision in advance and a five-minute argument afterwards.

And the arrangement on a printed card is fixed the moment it leaves the printer. If you need a game to be demonstrably fair — a fundraiser, anything with a real prize — the thing to keep is the record: the pool you drew from, the seed that produced the cards, and the caller's sheet. Between them they let anyone reconstruct exactly what was printed and exactly what was called, which is a far stronger answer to a complaint than assurance that the generator was random.