A count that keeps the hands busy so the rest of you can go somewhere — the mala, the tasbih, the prayer rope.
When
Any time. It's the same practice at any length.
How
Hold the beads, or just use your fingers. Choose one thing to repeat — a word, a line, a breath. Move one bead each time. When you reach the larger bead, stop or turn back; don't cross it. The count is the shape of the practice, never a score.
Try it
Nothing you type is kept. It floats off the page.
Where it comes from
Japa on a 108-bead mala in Hindu and Buddhist practice; dhikr on the tasbih in Sufism; the Jesus Prayer on a rope. Counting is universal. Grading the person is rare, and mature traditions leave readiness to a teacher, not a number.
Nothing achieved. That's the point.
The number has many explanations and no settled one. The beads don't mind.