Assertion: Around the 6th century BCE, the support for any particular sect of philosophy could grow and shrink over time.

Reason: Every time a new king came he would start his own religion and force people to adopt it.

The Assertion is correct but the Reason is incorrect.

We get a glimpse of lively discussions and debates from Buddhist texts, which mention as many as 64 sects or schools of thought. Teachers traveled from place to place, trying to convince one another as well as laypersons, about the validity of their philosophy or the way they understood the world. Debates took place in the kutagarashala – literally, a hut with a pointed roof – or in groves where traveling mendicants halted.  If a philosopher succeeded in convincing one of his rivals, the followers of the latter also became his disciples. So support for any particular sect could grow and shrink over time.