Ashta Matrika means the Eight Mother Goddess and is a concept that is believed to have expanded out of Sapta Matrikas. The concept of seven mothers, or sapta matrikas, became popular during the seventh-eighth century AD especially in the eastern parts of present day India.
The Sapta Matrikas are Brahmani, Maheshwari, Kaumari, Vaishnavi, Indrani, Varahi, and Chamunda. (This is the usual list but sometimes it changes depending on the region or the tantric text).
When the name of Goddess Bhairavi is added to the above list, then we have the Ashta Matrika.
As per the Ashta Matrika concept, the seven mothers, or Sapta Matrikas, appeared to nurse and take care of Kartikeya, son of Lord Shiva.
Bhairavi, the fierce form of Shakti, was added to the seven mothers to match the appearance of Shiva as Urdhvalinga Bhairavas.
The Ashta Matrikas are also known as Yoginis. The number eight is of great significance as Mother Goddess in Tantric cult appears in the multiples of eight.
We have eight yoginis, 16 Yoginis, 32 Yoginis and the famous 64 yoginis.
It is not always Bhairavi that is added to the list of seven to make it eight. Depending on the tantric cult and location. There is a change in the eighth yogini. Sometimes it is Mahalakshmi, Sri Lakshmi, or Narasimhi or Ambika.
More on Mother Goddess Shakti
Saptashrungi Mata – About Mother Goddess Shakti Worshiped at Saptashrungi
More on Mother Goddess Shakti
Saptashrungi Mata – About Mother Goddess Shakti Worshiped at Saptashrungi