Quick answer
Four is the standard and the only count the card assumes. Five and six are provided for in the rules through the bettor. Two and three are playable, but they are home conventions rather than League rules.
The honest answer has two halves, and most pages only give you the first. Four is the game: four seats, four winds, and the card assumes it. But five and six players are genuinely provided for in the rules through a role called the bettor, while two and three players are not, and every version of short-handed play you will read about is a house convention somebody invented. Knowing which of those you are doing is the difference between playing the game and playing a variant you agreed on.
Four, and why four
Four players, one to each wind: East, South, West and North. East deals, play passes to the right, and the deal rotates to the next player when the hand ends. The Charleston is built around four seats, since passing right, across and left only makes sense when there is somebody across from you.
This is also the count the card is written for. The hands assume four players feeding a discard pile and four racks competing for the same tiles, which is why short-handed games change the texture of the game even when they keep every rule intact.
Five or six: somebody bets
This is the part most people never learn, and it is the reason a fifth friend is not a problem. With five or six, four play and the rest sit out, but the player sitting out is not idle. They are the bettor, and the role is written into the rules.
The rotation is simple: after each game the player who just finished in the East seat steps out, and the next player waiting rotates in. Over a session with six, everyone sits out the same number of hands. For deciding who sits out first, the rulebook has players roll two dice, lowest totals out, with a re-roll to break a tie, though plenty of groups just agree it.
The bettor watches the Charleston finish, then looks at the racks and privately picks the player they think will declare mah jongg, or bets that nobody will and the hand ends in a wall game. The bet is placed after all Charleston passing is done and before play begins.
What the bettor may and may not do
Silence. The bettor may sit with the table and be sociable, but they cannot advise anyone, point out a missed joker exchange, note an error, or call a hand dead. Any interference voids the bet, which is the whole enforcement mechanism and the reason the role works at all.
If the bettor picked correctly, they collect alongside the winner: when the winner drew their own winning tile, both are paid at the doubled rate, and when the winning tile came off a discard, the discarder pays the higher amount while the rest pay the standard one. If the bettor picked wrong, they pay the winner what their chosen player owed. If they bet on a wall game and no one wins, they collect from each player at the lowest value the current card carries.
Two things follow from that. The bettor has real stakes, which is why sitting out is not a punishment, and the sit-out seat is the best place in the house to learn from, because you are watching four hands develop at once with a reason to pay attention.
Three players
Three works, and the tile count does not change: full set, thirteen tiles each, fourteen to the dealer. What changes is the Charleston, because with three seats there is no across. Groups usually pass right, then left, then right again, keeping three rounds so the same number of tiles move.
It is worth knowing that the official short-handed procedure is different from what most tables actually do. The published three-handed method deals differently and skips the Charleston entirely, and the people who document it say plainly that most American players reject skipping it and invent a passing round instead. Neither is wrong, but they are not the same game, so agree which one you are playing before the deal rather than halfway through it.
Expect the wall to last longer per player, more draws each, and fewer wall games than you get with four.
Two players
Two is possible and it is openly improvised: there is no League two-handed game. Use the full set, deal thirteen each and fourteen to the dealer, and either pass three tiles across once or twice or skip the passing altogether. Passing across is the better choice, because it keeps the decision that shapes every hand.
Wall games become much more common, since fewer tiles change hands and any direction needing a called set can run out of wall. Choose more forgiving directions than you would with four.
Two-handed play is a poor substitute for a real table and an excellent practice format for the one skill that matters most, which is reading a deal and naming where it points.
What to agree before you start
If you are four, nothing: play the game. If you are five or six, agree that you are using the bettor properly, including the silence rule, because a chatty sit-out ruins hands and the penalty only bites if everyone knows it exists.
If you are two or three, agree the passing convention out loud before the deal, and agree it again if somebody new joins. Short-handed mahjong has no single right answer, and almost every argument about it is really two people playing different house rules without knowing it.
Put it into practice
A fresh coached hand every morning, the coach on call for your own tiles, all ten lessons and the 14-day course. $14.99 a month, cancel anytime.
Whatever your count, the hand in front of you still has to be read. Mei, our AI coach, takes your tiles and says which direction they point, what the backup is, and what to pass, which is the skill short-handed games are worst at teaching.