What's the average number of downloads per day?

That is a difficult question to answer.
Contributors who submit a large volume of content likely see a high number of downloads.
You should aim to submit at least 1,000 items.

Also, if you choose a theme that is open to everyone, you will face stiff competition, which likely means fewer downloads.
There is a difference in sales potential between a theme with 10,000 existing listings and one with only 1,000 results.
The theme you choose when creating your assets is also crucial.