“Initially, only one soul was created, the soul of Adam HaRishon”
(Baal HaSulam, 600,000 Souls)

“Man has a body and a soul. The body is called a Kli [vessel] of the will to receive pleasure and knowledge. The soul is regarded as the power of bestowal that exists in man. And man has a body and a soul.”
(Rabash 381. The Lord Hears the Poor)

There are no new souls
“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, for each time they clothe a new body and a new generation.
Therefore, 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 his 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 Purpose of the Soul
“The purpose of the soul when it comes in the body is to be rewarded with returning to its root and with Dvekut [adhesion] with Him.”
“It is written in Tree of Life, “The worlds were created only to disclose the names of the Creator.” Thus, you see that since the soul came down to clothe this filthy substance, it could no longer adhere to its root, to its own world, as before it came to this world. Rather, it must increase its stature 620 times, as it previously was in the root. This is the meaning of the entire perfection.”
(Baal HaSulam, Letter 17)
 

The soul is only acquired by equivalence of form
“The body is born with a nature of self-love, which is disparity of form from the Creator, whom we attain as only giving. Thus, one should cleanse one’s body and come to equivalence of form so he, too, will want to do things that are only to bestow. By that, he will be able to reach this high degree called Neshama. This is why we always speak in terms of body and Neshama [soul].”
(Rabash, Sometimes Spirituality Is Called “a Soul”)
 

The correction of the soul
“It follows that each one is born with only a tiny part of the soul of Adam HaRishon. When he corrects his part, he no longer needs to reincarnate.”
(Rabash “What Is the Degree One Should Achieve in Order Not to Have to Reincarnate?”)
 

Reincarnation of the Souls
"Come and see what is written, 'If you do not know, O fairest of women': the Creator answers the soul, if you came, and did not look into the wisdom before you came here, and you do not know the secrets of the upper world, leave, you do not deserve to enter here without knowledge… Meaning you will be reincarnated a second time into the world.”
(Book of Zohar, Song of Songs, Part 10, point 456)
 

What should I learn from my current incarnation?

“Therefore, we must ask, Why then did the Kabbalists obligate every person to study the wisdom of Kabbalah? Indeed, there is a great thing about it, which should be publicized: There is a wonderful, invaluable remedy to those who engage in the wisdom of Kabbalah. Although they do not understand what they are learning, through the yearning and the great desire to understand what they are learning, they awaken upon themselves the lights that surround their souls.
This means that every person is guaranteed to finally attain all the wonderful attainments with which the Creator contemplated in the thought of creation to delight every creature. And one who has not been awarded in this life will be granted in the next life, etc., until one is awarded completing His thought, which He had planned for him.”
(Baal HaSulam “Introduction to The Study of the Ten Sefirot”, p.155)