Why do you chant the Hare Krishna Mahamantra and why specifically 16 rounds?
There are four yugas i.e. Satya, Treta, Dwapara and Kaliyuga and for every yuga a specific process has been recommended in the scriptures for self realization.
Satyayuga – Meditation
Tretayuga – Yajna (sacrifice)
Dwaparyuga – Deity worship
Kaliyuga – Harinaam sankirtan (congregational chanting)
The scripture Kali Santaran Upanishad mentions the specific Mahamantra (comprising of the Holy name of the Lord) i.e. “Hare Krishna Mahamantra” to be chanted as the only process required for self realization in Kaliyuga.
Moreover, this Mahamantra has been given by Lord Chaitanya who is none other than Lord Krishna himself. So as followers of Lord Chaitanya Mahaprabhu we chant this Mahamantra.
Srila Prabhupada, the founder Acharya of ISKCON always recommended that we should chant the whole day. But seeing the restless nature and hectic lifestyle of the present yuga he mercifully reduced it to 16 rounds.