Plurality of mind would result in lack of understanding between one mind and another

Verse 4.21

चित्तान्तर दृश्ये बुद्धिबुद्धेः अतिप्रसङ्गः स्मृतिसंकरश्च ॥२१॥

citta-antara-dṛśye buddhi-buddheḥ atiprasaṅgaḥ smṛti-saṃkaraḥ ca ||21||

Another cognising mind being assumed there will be no end to such assumptions and confusion of memory.

Plurality of mind would result in lack of understanding between one mind and another, leading to utter confusion and madness. Patanjali therefore concludes that mind is one and cannot be many.

As a tree has many branches, all connected to the trunk, the various wavelengths of thoughts are connected to a single mind. This mind remains pure and divine at its source in the spiritual heart. When it

branches from the source towards the head, it is called created consciousness,, which, being fresh, is untrained and uncultured. 

The moment it comes into contact with objects, it becomes tainted, creating moods in the thought-waves. These moods arc the five fluctuations and five afflictions .These moods should not be mistaken for a plurality of minds. The mind remains the same, but moods create an illusion of several minds. 

If the minds were really many, then each would have its own memory and intelligence.

This becomes preposterous. Just as a room fitted with mirrors puzzles the onlooker, the idea of many minds causes confusion and absurdity.

The practice of yoga disciplines and cultures the consciousness of the head, by which it perfects the art of analysis (savitarka) , j udges precisely, experiences unalloyed bliss (ananda) , becomes auspicious and moves towards mature intelligence (consciousness of the heart) and unalloyed wisdom (ritambhara prajna) .

PATANJALI YOGA SUTRAS