How many people can access podcasts on iTunes and Google Play?

Potentially any user with a smartphone.

You have to advertise your product. As David says, anyone with a phone. I do not look at 0.001% of what is out there because:

1. I have no need.
2. I have all I want for my iPhone at this point.
3. Advertising is untargeted  "You just purchased X - wanna buy more?"; You looked at X - wanna buy it"  

So you need to find a way to make people aware of what you have, make it worth purchasing it if you are selling

Anyone having access to iPhones can access iTunes and anyone having android can access Google play.

I was hoping to hear numeric estimates to the max size.

There are millions of people who can do this, so no practical numerical estimate.

You buy a domain and put up a website, how many potential people could visit your website? Potentially billions, in reality you could be the only person to visit it.

thanks

thanks