The answer is that it depends on the text, and the modern assumption of seven is a simplification. Indian tantric and yogic literature describes systems of four, five, six, seven, nine, twelve and more centres; the Kubjikamata Tantra and other early works give different sets, Buddhist tantric systems generally work with four or five, and Tibetan practice normally uses four principal chakras with a somewhat different distribution. The six-plus-one scheme that Purnananda gives, six centres from the base to the brow plus the thousand-petalled sahasrara above them, became dominant in the west because Woodroffe translated a text that uses it. Geoffrey Samuel and other historians of Indian religion have traced this variety in detail. Anyone who has been told that the seven chakras are an ancient and universal Indian teaching has been told something a little too tidy to be true.
Buddhist tantric literature offers a genuinely different structure rather than a subset of the Hindu one: texts associated with the Chakrasamvara Tantra generally work with four or five centres, head, throat, heart, navel and sometimes the genitals, tied to a scheme of channels and subtle drops rather than to seven chakras and their elements, and used differently, chiefly in completion stage practices aimed at generating the experience of clear light. An earlier Hindu source than Purnananda's, the Kubjikamatatantra, compiled around the tenth or eleventh century, likewise describes a five centre system, further evidence that the six-plus-one scheme was one solution among several before Woodroffe's translation made it the default.