Arturia BeatStep Pro MIDI/Analog Controller and Sequencer Kit with CV/Gate Cables (Black) - Manual (Page 110)

10.7.3. Pattern length (Polyrhythm Off)
When the Polyrhythm [p.63] parameter is disabled you can change the length of the entire
Drum pattern by clicking the white line and dragging it to the left or to the right:
Here’s how the pattern length looks when the length is set to 12:
The patterns can be up to 64 steps long, and the unused steps are darkened because they
are outside the pattern length area.
The steps are numbered along the bottom of the window:
105 Arturia - User Manual BeatStep Pro - MIDI Control Center