प्राचीन वैदिक ग्रंथों में वर्णित ऐतिहासिक शब्द 'सप्त सिंधु' निम्नलिखित में से किसे संदर्भित करता 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?