A number of late John Cage pieces are collectively called the Number Pieces. It is 40 pieces written between 1987-1992.
They are called Number Pieces because of the naming of them. They are all named a spelled out number and optionally followed by a sequence number. The spelled out number represents the number of player that should perform the piece. For instance the piece Six is the first piece for six performers and the piece Four3 is the third piece for four players.
In these pieces Cage often uses what is called time brackets to denote time.The notes in a measure does not have duration, instead each measure is marked with times. Either fixed, e.g 1.00 – 1.37 (start the measure at 1.00 and end at 1.37) or flexible 1.00 <-> 1.15 – 1.30 <-> 1.40 (start between 1.00 and 1.15 and end between 1.30 and 1.40).
The pithes in the pieces is often chosen via I Ching, a method to choose by chance that he developed when we was composing Music of Changes.
The instrumentation and method of playing is also many times chosen by the performer.
I find it interesting that although Cage leaves a lot of freedom to the performers he seems to fulfill the purpose and spirit of the pieces when they are performed. The resulting music is, like much of his music, very calm and meditative.