While originally intended for 2 players (which is still the best way to play it), any number of people can play.
The gameplay is simple. A deck of 52 cards is used. The dealer shuffles the deck and places it on the table face down. The player to the left of the dealer is the first to go - he picks up a card from the deck, taking care not to show it to anyone. Now he can either discard it on the table, face up, or declare it a spade and put it in front of him, face down. If the latter happened, any other player can, but is not obliged to, challenge the first player by opening the card. If it's not a spade (ie. the player attempted bluff), the first player is out of the game; if it is a spade, the second player is out. When only one player is left, he is declared the winner; with 2 people playing, the winner will always be determined on the first challenge.
The only other way to win is to collect a pre-determined number of spades - seven spades in the two-player version. The last (seventh) spade has to be a real spade, and is put face up, not face down. The person to do this is declared the winner.
Anyone can challenge a person who claims to have a spade, but only immediately after the card is put down. When the next player picks up a card, you are no longer allowed to challenge the previous player. Only the last card a player put down can be challenged, not the other ones.
When more than 2 people are playing, the number of spades to collect is adjusted so that it's always possible to win without bluffing. With a single 52-card deck, the number of spades to collect is:
for 2 people: 7
for 3 people: 5
for 4 people: 4
for 5 people: 3
To increase the number of spades, or to allow more than 5 people to play, more than one deck can be used. The number of spades to collect is then multiplied by the number of decks used.
Optionally the following rule can be added when more than 2 people are playing: if a person challenges another player and proves him a liar, he gets a free turn. This promotes challenging and makes bluffing a bit more expensive.