According to this article ref here from Eötvös Loránd University (Hungary)
An important difference between schools of Indian philosophy that
recognize mokṣa(liberation) as an end is the accepted number of souls.
In Buddhism there is no separate soul to be liberated. In Advaita
Vedānta, there is one common world-soul, and individuality is a
function of the material world only. Sāṅkhya adduces three arguments
to prove that there is a separate puruṣa for each individual: (1)
Birth, death and the personal history of everybody is different (it is
determined by the law of karma, according to our merits collected in
previous lives). If there were one puruṣa only, all bodies should be
identical or at least indistinguishable for the function of the self
orpuruṣa is to be a supervisor of the body. But this is clearly not
so. Hence, there must be a plurality of distinct puruṣa-s. (2) If
there were only one puruṣa, everyone would act simultaneously alike,
for the puruṣa is the supervisor of the body. But this is clearly not
so. Hence, there must be a plurality of distinct puruṣa-s. (3) If
there were only onepuruṣa, we would all experience the same things.
However, it is evident that the opposite is true: our experiences are
inherently diverse and private, and they cannot be directly shared.
Hence, there must be a separate puruṣa for us all.
Swami Vivekananda taclked the same question and accepted a the same view as the Vedanta school. ref here
...Now comes the more important point of difference. Can there be more
than one Purusha? The Purusha, we have seen, is omnipresent and
infinite...All difference is due to time, space, and causation. These
are the constituent elements of the mind. No mentality is possible
without them. You can never think without time, you can never imagine
anything without space, and you can never have anything without
causation. These are the forms of the mind. Take them away, and the
mind itself does not exist...
From the Samkhyakarika ref here
Verse 18 of the Karika asserts that many souls must exist because
numerous living beings are born, die and exist; because qualities
(Gunas) are operating and affect everyone differently; and because
everyone is endowed with instruments of cognition and action.
There seem to be no modern literature nor modern authors of the Samkhya philosophy defending why there should be an plural number of purushas. Unlike Vedanta or Buddhism Samkhya does not accept the possibility of the same consciousness operating simultaneously in different places in time and space or in other words being independent of mind-space-time. It seems like an old belief or flawed philosophy that also has been victim of impermanence.
"Achintya-bheda-abheda-tattva" (The Supreme Lord is simultaneously and
inconceivably one and different from His creation)