Some sources speak of 51 pieces of Sati rather than 52 pieces.
Singh, explains why 51 is reached in Vindhyachal yantra:
Rotation of the cosmic rhythm from point to triangle (3 edges) can yield a combination of 8, 16, 24…angles ultimately merging into a circle again at infinity. This idea  indicates the sense of expanding universe, commonly referred to in Hindu and Buddhist mythologies as mandala. The total number of these edges (3+ 8+16+24) comes to 51 identified to total number of alphabets in Sanskrit/Devanagari and also the Shakti Pithas.  

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