In very olden days, 8 great Female Goddesses Shaktis emerged on the cosmic soul of the Principle Dieties and formed Kali Durga, the Universal Shakti Power, These were the grand Mothers (Ashta Matrikas) of all subsequent Yoginis. As outlined by Kaula Tantra, these eight manifested each in turn into 8 Divine Shaktis, thus giving you the 64 Tantric Yoginis.
An examination of the ancient Tantric tradition reveals a particular sanctity assigned to the quantity eight. The 8 mother faculties (tatvas) of the manifested universe, the eight directions with four cardinal and four intermediate points (digbandahs), the eight miraculous yogic powers (siddhis), eight "limbs" of Yoga (astanga) eight forms of the Divine Mother (matrikas) and eight primary mystic symbols (mudras) are a few examples. The square of eight, or sixty-four, occupies an even more profound position in the field of Tantra vidya which, from the point of view of the practitioner, originally identifies the sixty-four Tantric Yoginis.