मत्स्य महाजनपद आधुनिक राजस्थान के किस शहर के आस-पास के क्षेत्र में स्थित 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?