Basically, one's only constraint is the number of cards available. For a good game you want to have a realistic possibility of high doubles. To that end, even with four players, I would advise two decks and once you're hitting more than six consider adding in a third but it's really a matter of taste. Bear in mind that more players also requires more space and you need to be able to hear everyone too.
If you want to play with fewer than four players? Well, three man scum and two man scum are entirely possible. The thing to bear in mind is that you know a lot more about the opponents' cards, which means you either need to be a lot luckier or a lot better.
To be the first player to have no cards in hand, i,e, become the President.
Standard Pack of Cards... my personal recommendation, as above, is two.
You want to keep the jokers in as well so if you know a brand of cards that has four jokers in it? All the better. (See, for instance, Cardinal card decks in New Zealand.)
Deal out all the cards, face down, in any direction, so that each person has a roughly equal number of cards (having a few less is both advantageous and disadvantageous).
Pick up your cards and order. I personally prefer to have the low cards on the left and the high cards on the right but the important thing is that one's hand is ordered in a manner one understands. Order of cards, low to high, as follows:
3 4 5 6 7 8 9 10 J Q K A 2 Joker
The 3 of Clubs always starts the game: it may be played individually or with any other number of threes. Play proceeds clockwise. If more than one deck of cards is being used, the fastest three of clubs starts (first to play, in other words).
To play in a specific trick one must play a) cards of a higher worth (i.e. you can't play a 9 on a 9) and b) the same number of cards as played at the start of the trick. If one cannot play, one is obliged to pass. Once one has passed, one is out of the trick. The trick ends when all players have passed. The cards from the trick are left in the middle/moved to the side.
The player of the cards passed on starts the next trick afresh. That is, if the previous one ended with trip two (i.e. three twos) then the next trick may start with, say, two threes no worries. All passed players from the previous trick are back in, until they pass again.
The set, as it were (if a trick is a game), ends when all players bar one have no cards left. The first two payers to have no cards are, respectively, termed the President and the Vice President. The last two are, respectively, the Vice-Scum and the Scum (i.e. second to last and last, in that order). Any players in between are known as neutrals.
Before the next set begins, the Presidents and Scums exchange cards. The Scum is obliged to give the President whatever two cards the President desires (although the President is not allowed to keep asking for cards in order to discover all the cards in the Scum's hand), and receives whichever two cards the President wishes to give the Scum. This is the same for the Vices, albeit with only one card involved. Typically the President demands the highest card and gives low cards back.
The next set begins with the three of clubs as before.
The match ends when the participants decide to end it. As we developed these rules at school, this was generally when the bell rang. There is no overall winner but there is no reason why a points system could not be developed.
- Passing - one is always able to pass. At no point in a game of President is one obliged by the rules to play. However, whether one passes for strategic/tactical reasons or because one has no valid cards to play, one is forced to sit the trick out from that point forth.
- Consecutive - this special rule, typically known as "consec", occurs whenever a run of three consecutive numbers occurs. When consec is in operation all players must continue to play the same number of cards but can only play consecutive cards. That is, a joker will not beat a 9 in consecutive as only a 10 can be placed on that 9. The trick continues under consecutive until all bar one players have passed.
- Eight Below - the eight is one of three exceptions to the "must play a card greater than" rule. Whenever an eight is played, the next card placed on the pile must be lower than an eight, although the card after that may be higher. Thus, 8, 7, 9 is valid but 8, 7, 4 is not. The run 6, 7, 8 will be consec as consec takes priority, but the run 7, 8 may not be followed by a 9. Eight Below does not affect the "same number of cards" aspect... that rule is immutable.
- 69 - whenever a nine is played on top of a six, the 69 rule applies. This means that to remain in the trick a player must play any two face cards (i.e. J, Q, K) regardless of whether or not they remain a pair. 69 is another exception to the "higher than rule" in that if you have the run 6, 9, JK you can then follow up with JQ (or any other combination of faces). A trick operating under the conditions of 69 ends only once everyone passes on either the last card played (frequently, this is the 9). If the run is 66, 99 (double 69) then four face cards (any four) must be played, if the run is 666, 999 (triple 69) then six face cards must be played and so on. In this way the immutable rule of "same number of cards" is preserved (69 requires two cards, therefore two face cards... even though that starts from a singles trick).
- KKK - if three kings are played (i.e. trip king) then all subsequent players must play three red cards (any three red cards). KKK only exists with triples.
In President, the luck of the draw is important but it is not insurmountable. While it is unlikely that a hand stack top to bottom with face cards, aces, two and jokers (lowest card Jack) will lose, it is possible. This is due to rules like eight below and consecutive which mean that ludicrously high hands can be forced to pass with no opportunity to play any cards. The immutability of the number of cards also means that it is impossible to beat a single joker if playing singles, double joker if playing with doubles and so on. This means that a ludicrously high hand can be forced to lose a trick. However, it is unlikely that such a ludicrously high hand will be dealt out (especially if the cards in the pile are not shuffled after the end of a "set"; this breaks up any doubles and runs of high cards).
What is more likely is that a player, particularly a scum, ends up with a very low hand (even when playing with two decks). This situation is recoverable as such players tend to have a number of lower doubles; so if one waits long enough and then wins a trick, it is often possible to win several in a row (as most players will have exhausted their doubles/triples etc earlier). Players faced with such hands will look to utilise eight below, 69 and consecutive.
The value of various rules changes as the game goes on. In the early stages, consecutive is an important rule as it frequently allows a couple of players to use up a lot of singles quickly. In the later stages of a game, consecutives are harder to play and to create due to fewer cars. My gut also says late consecutives are either high or very low (more J, Q, K or 3,4, 5 than 5, 6, 7 for instance). An early 69 is an opportunity to leverage a hand stacked with faces whereas a late game 69 often allows scums a chance to win a trick as people have often got only one face card (or a high double they don't want to waste). An early stage Eight Below is generally wasted (see above) as players are almost guaranteed to have low cards: in the late game this is much less likely (which is why I keep hold of sixes and sevens even late on). KKK is unlikely to appear near the end of a game: people who play KKK still want to have three reds to dispose of themselves.
In general, players will play low first and then try and build to the higher cards. This manner of playing is designed to maximise the number of cards played in a trick. Sudden jumps, eg. 5 to J, are generally signs of inexperience or aggression. It is much easier to play aggressively as a President (of either kind) as such players are more likely to have higher hands. In this sense, I personally suggest that once you are familiar enough with these rules (or, indeed, any other rule system) you start to predict from the very start, before even the three of clubs is played, whether or not your hand can win. If you don't think you can win from the start, it is generally better to play to avoid becoming a scum, rather than to become President. That being said, any hand can win. With hands that don't look so good at the start, winning is a matter of timing one's push: move too soon and you're often left stranded with fairly low cards.