Monetizing my podcast
At what level of downloads per episode can one begin to try and monetize the podcast?
You cannot charge for podcasts in the iTunes Store/Apple Podcasts.
It used to be possible to password-protect a feed for subscribers in iTunes, which would handle a password request set up by the server. I don't know whether this is still possible, and I seriously doubt whether the facility is available in the Podcasts App. it's not something I can very well test, but you could try a test yourself, by setting a a password-protected folder on your server. However, as I say. either way this can't be done in the Store or Apple podcasts Browse, only for subscribers.
As to advertising content, Apple say: 'Spam content or other content created with the sole purpose of advertising or marketing products, content, or services outside of Apple Podcasts is prohibited.'
Whether this would apply to a podcast which had normal content but carried the odd advert or sponsorship I don't know. You would have to ask Support about that. Go to https://itunespartner.apple.com/en/podcasts/overview and click the 'Contact Us' link at the bottom of the page.