a. If a soul goes to a new body, where does a new soul come for increased world population?

First, Soul is not Atman.

Think of Atma (or Soul as in this question) as the life energy within you that moves from one body to another body in the cycle of life and death that we call Samsara. This life energy is a part of the infinite,  unbounded energy that prevails the entire cosmos (Parmatama). When one talks about life energy, it is unbounded and not in terms of numbers. 

People ask when more births are happening than death, does that mean souls are coming from other life forms (animals, plans, etc.) and the total aggregate number of souls is constant on Earth?

Please know that numbers are only for the physical world, the life force (atma) knows no such physical limit and its arithmetic as the fundamental life force is unbounded and permeates the entire existence. 

I have one lit candle. Five people come by and light their candles from my one light. Where have the new flames come from? The rules of physical mathematics do not apply to the spiritual realm.

Can we reach infinite by counting 1, 2, 3 to 100, to 1000, to millions and so on? We will keep counting endlessly but will never reach the Infinite. So the question of the finite number of souls on earth would not be a valid one as soul is an infinitesimal part of this unbounded infinite energy and we can not count in terms of numbers. 

Source: Sadhguru Satsang

Other reading material:

Relevant videos: Swami Nikhil Anand Is the soul of an ant bigger than the soul of a whale?,

Contributor: Vikas Gupta

Location of this post (16a): https://www.hinduspeakers.org/ufaqs/if-a-soul-goes-to-a-new-body-where-does-a-new-soul-come-for-increased-world-population/