In our world, there are no new souls the way bodies are renewed, but only a certain amount of souls that incarnate on the wheel of transformation of the form, because each time they clothe a new body and a new generation.
Baal HaSulam, “The Peace”
Although we see the bodies changing from generation to generation, this is only the case with the bodies. But the souls, which are the essence of the body’s self, do not vanish, to be replaced, but move from body to body, from generation to generation. The same souls that were at the time of the flood came also during the time of Babylon, and in the exile in Egypt, and in the exodus from Egypt, etc., until this generation and until the end of correction.
Baal HaSulam, “The Peace”
With regard to the souls, all generations since the beginning of Creation to the end of correction are as one generation that has extended its life over several thousand years, until it developed and became corrected as it should be. And the fact that in the meantime each has changed its body several thousand times is completely irrelevant, because the essence of the body’s self, called "the soul," did not suffer at all by these changes.
Baal HaSulam, “The Peace”
The eternal soul of life that the Creator had blown in his nostrils, only for the needs of Adam ha Rishon, has departed because of the sin of the Tree of Knowledge. It acquired a new form, called “Sweat of Life,” meaning the general has been divided into a great many particulars, tiny drops, divided between Adam ha Rishon and all his progeny through the end of time.
It follows, that there are no changes in the acts of the Creator, but there is rather an additional form here. This common Light of life, which was packed in the nose of Adam ha Rishon has expanded into a long chain, revolving on the wheel of transformation of form in many bodies, body after body, until the necessary end of correction.
Baal HaSulam, “Introduction to the Book, Panim Meirot uMasbirot,” Item 22