Overtone singers employ very high harmonics, around the twentieth to twenty-fifth.

Evidence

The underlying drone note is F#2 with an f0 of 93 Hz. Since the underlying vowel is fixed, the overtones can be seen as any harmonics whose level (darkness of marking) varies, and in this case, it is approximately the twentieth to the twenty-fifth harmonics that are employed. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1661