Assertion: All the Mahajanpadas of Ancient India were ruled by kings.
Reason: Some Mahajanpadas were known as ganas or sanghas, and these were oligarchies where power was shared by a number of men, often collectively called rajas.

The Assertion is incorrect but the Reason is correct.

While most mahajanapadas were ruled by kings, some, known as ganas or sanghas, were oligarchies where power was shared by a number of men, often collectively called rajas. Both Mahavira and the Buddha belonged to such ganas. In some instances, as in the case of the Vajji sangha, the rajas probably controlled resources such as land collectively.