In yoga, it has been mentioned that there are 72000 nadis in human body.
Are they physical or non-physcal?
The nadis in yoga and tantra shastra are non-physical/subtle. See here for a detailed reference.The nadis are the intersection point of vital forces in the spiritual/astral body. As such, they cannot be seen to the naked eye.
As a side, even some ayurvedic physicians and astrologers use the term nadis synonymously to the pulse and nerval plexuses. These are physical objects.