आरम्भिक वैदिक साहित्य में किस नदी का सर्वाधिक उल्लेख मिलता है?

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?