I read somewhere that souls are never created nor destroyed. How is that possible? If souls are never created nor destroyed , how did all of us come into existence and how is the population of the world increasing?
God created the universe as thought and as "leela", as per scriptures. In his creation there are many forms of energy depending upon its manifestation, it is can be birds, animals or human beings or stones also. These forms can interchange just like a magician doing a trick, however, in god's creation there is no magic everything goes as per defined nature's laws (nature laws= simplified term for all writings in vedas & upanishads).
In ancient days, there were many birds, animals which became extinct as man invented industrialization and destroyed them. However, soul can/may return as human beings. In short there is mass balance & equilibrium in mother nature.
To understand the creation and understand the soul, one need to read holy scriptures, then only further discussion can be held.