In the Synth Details slide-out pane of Logic Pro Sampler:
Choose the number of voices from the Mode pop-up menu.
In Poly/Unison mode, each note you play is effectively doubled. These two voices are heard when you play each note.
A polyphonic instrument, such as an organ or piano, lets you play several notes simultaneously. Brass or reed instruments are monophonic, which means that you can play only one note at a time. You can choose an appropriate keyboard mode for the type of instrument that is loaded. You are also free to use a monophonic mode for polyphonic instruments such as pianos or guitars, which allows playing styles that are not possible with these polyphonic instruments.
Choose Legato, Mono, or Poly from the Mode pop-up menu.
In Legato mode, only one note can be played at a time. The envelope generators are retriggered only if you play staccato—releasing each key before playing a new key.
In Mono mode, sample playback and envelopes are always retriggered regardless if you play staccato, or in a legato style—play a new key while holding another.
In Poly mode, you can play several notes simultaneously.
Note: In many classic monophonic analog synthesizers, the behavior in Legato mode is referred to as single trigger, while Mono mode is referred to as multi trigger.
Drag vertically in the Polyphony field to determine the maximum number of notes that you can play.
When you play the keyboard, the Used Voices field shows the number of voices in use. The number of voices actually used for each note you play increases with the number of layered sample zones.