Analysis of information sources in references of the Wikipedia article "History of Buddhism" in English language version.
Lamaism (Vajrayana/Tibetan/Tantric) — 20,000,000
In the past it was not uncommon for historians to conflate the vast space thus outlined with the oppressive realm described in the Arthashastra and to posit one of the earliest and certainly one of the largest totalitarian regimes in all of history. Such a picture is no longer considered believable; at present what is taken to be the realm of Ashoka is a discontinuous set of several core regions separated by very large areas occupied by relatively autonomous peoples.
Magadha power came to extend over the main cities and communication routes of the Ganges basin. Then, under Chandragupta Maurya (c.321–297 bce), and subsequently Ashoka his grandson, Pataliputra became the centre of the loose-knit Mauryan 'Empire' which during Ashoka's reign (c.268–232 bce) briefly had a presence throughout the main urban centres and arteries of the subcontinent, except for the extreme south.
Lamaism (Vajrayana/Tibetan/Tantric) — 20,000,000