सिद्धार्थ को किस नदी के किनारे पीपल वृक्ष के नीचे ज्ञान प्राप्त हुआ था?

In Ancient Indian Statecraft, Kautilya's 'Rajamandala' theory primarily deals with:

The First Buddhist Council was held at ______.

Which ancient Indian text contains references indicating that population counts or census-like records were maintained in ancient India?