What is a soul?
As per Brihadaranyaka Upanishad verse 3.7.23,
नान्योऽतोऽस्ति द्रष्टा, नान्योऽतोऽस्ति श्रोता, नान्योऽतोऽस्ति मन्ता, नान्योऽतोऽस्ति विज्ञात, एष त आत्मान्तर्याम्यमृतः, अतोऽन्यदार्तम्;
“He cannot be seen, but he is the seer; He cannot be heard, but he is the Hearer; He cannot be thought, but he is the Thinker; He cannot be known, but he is the Knower. He is the Internal Ruler, your own immortal Atman. Everything else is destroyed except the Atman”
The subtle power who runs the body, senses and mind, when he leave them, this living body is called a dead body, he is known as Atman (soul)
Can we say number of atma (souls) is always the same ? If so how many of them exist?
According to Vedanta there is no separate Atman’s, there is only one single Atman.
Let take a example of dream, where you see thousand of people are they different souls ? No, the perception belongs to you, you are the seer and other person are only objects. They are not conscious because you are the one who is conscious of your dream.
Similarly here in this world you are the conscious being, the knower of world and others are objects for you. All other billions of people are the objects of perception for you.
If you unable to understand the above point, than understand single Sun (Atman) whose reflections is seen in multiple pots ( Bodies)
Soul can neither be created nor be destroyed, but it also said that everything that is created has to be destroyed how is it possible?
This statement is said for physical bodies and no for the soul. As bodies can be multiple, but the Atman is never created or destroyed.
जीवापेतं वाव किलेदं म्रियते न जीवो म्रियते
“This body without the Atman dies, Atman never dies” – Chandogya Upanishad 6.11.3