Prakṛti is imagined as a singular entity (all of nature).
By contrast, Puruṣa is thought to be multiple. That is, Puruṣa is different in each person, because each person has a different temperament or “spirit.”
So we can speak in terms of multiple “puruṣas” (each person as a manifestation of an encompassing cosmic Puruṣa) (compare to ātman and brahman).