Answers to Mahendran’s questions are embedded in the following statement itself:
“Since humans have the control of certain animal populations can I also assume that there will be countless number of gandhabbas are waiting to get in to the wombs of those animals?”
Yes. There are essentially an innumerable animal gandhabbas in the para loka waiting for suitable conditions to be born.
There are also many, many more human gandhabbas waiting for suitable wombs than those with human bodies like us. We can get a rough idea of how many gandhabbas must be waiting for a womb, because in between births in rebirth stories is several years, and in some cases many years. This is why one needs to be grateful to one’s parents for proving that rare opportunity.
The world is much more complex than we can even imagine. The following video gives an idea of the complexity:
There are as many creatures on your body as there are people on Earth!