The ultimate goal in chess is to checkmate your opponent, which means to put his king in check with means of escape. A checkmate usually occurs at the end of a game when both sides have just a handful of pieces in play. It is possible to execute a checkmate against a lone king even if you have just your king and one other piece, but that piece must be a rook or a queen.
Against a lone king, the minimum number of pieces an opponent needs to force checkmate is two, including his king. Even if the other piece is a single pawn, checkmate is possible. The player with the advantage will need to advance the pawn to the eighth rank and promote it to a more powerful piece.
Queens and Rooks
A queen, rook and king combination is the strongest way to checkmate a lone king. The queen or the rook can block a lone king on a single rank or column and corner it. The king can protect the queen or rook when it checks the lone king, affecting checkmate. Two rooks are also able to checkmate without the aid of their king.
Because a bishop is restricted in movement by colour, a single bishop cannot checkmate a lone king, even when aided by its own king. It is possible for a bishop and knight, aided by a king, to checkmate a lone king. This is a rare situation and requires more than two pieces.
Because knights cannot threaten the squares directly surrounding them, one or both cannot be used to checkmate a lone king even when aided by a king. In the unlikely event that a pawn is promoted to a knight, then three knights and its king can work together to checkmate a lone king. This, however, is rare and far exceeds the minimum number of pieces needed for checkmate.