Subscribers vs Podcast Downloads
Is there a common subscriber to podcast download ratio?
As far as subscribers vs uniques, there's no one good answer to explain that (as far as a common ratio). We generally don't compare the two and treat them totally separately.
We've seen more and more podcast apps not encourage subscriptions, or not even send the subscriber numbers, so tracking subscribers becomes completely useless as a metric for many (Pocket Casts, Castro, Stitcher, TuneIn, Spotify, Apple Podcasts).
Some apps default to streaming, though this still counts as a download because streaming just does partial downloads. iTunes (the desktop app) on the other hand, even though it auto downloads new episodes, doesn't work in the background, and many people don't leave that app open 24/7. The downloads do eventually get counted.
Also, there are RSS clients that send subscriber data, but don't auto download anything (think regular desktop or browser based news readers).
For comparison, I have a podcast of my own with around 1,200 subscribers, yet after 7 days of a new episode being released, I get around 100 unique downloads.