It only measures the voltage between 2 electrodes, the 3. electrode being ground.
Technically, Dr. Scott Hardens PhD. Sound Card ECG is a:
(3 electrode) 1 lead ECG system
According to the scientific papers, reference devices for sports sensor accuracy validation have up to 3 channels (or leads) with up to 12 electrodes
for instance the
- Schiller Medilog AR12 Plus Holter Recorder:
https://www.medicaldevicedepot.com/Medilog-AR12-Plus-p/1.35001.htm
used in the paper:
Rahel Gilgen-Ammann, Theresa Schweizer, Thomas Wyss, Eur J Appl Physiol. 2019 Jul;119(7):1525-1532.,
RR interval signal quality of a heart rate monitor and an ECG Holter at rest and during exercise
has 3 channels with up to 7 electrodes
2. or the
2. Vyntus™ ECG 12-lead PC-ECG:
https://gha.health/en/produkt/vyntus-ecg-12-lead-pc-ecg/
used in:
Paul Hough , Mark Glaister, Adam Pledger
Journal of Physical Activity Research. 2017, 2(2), 112-116. DOI: 10.12691/jpar-2-2-8
The Accuracy of Wrist-worn Heart Rate Monitors across a Range of Exercise Intensities
http://pubs.sciepub.com/jpar/2/2/8/index.html
has 12 ECG electrodes.