According to my experience, an optimized site with static content in a good niche can bring $3 - $15 a day from 100 visitors a day. But you need a quality site to get high CTR and high CPC keywords...
Mainly it depends on the focus of your visitors. You may only need 3-4k visitors with rich content and valuable/interested visitors, but may need 10x that if your visitors are social media/less interested visitors.
It depends on the content on your blog. A good content site can always grab the attention of the reader. 500 - 1000 visitors per can surely give you $20 per day especially if your targeting high-paying keywords.
Its a site with little content, you need spiders to crawl. With little content visitors will come and go without conversion. Is extremely difficult to get 20 a day but the niche is good.
As many have said it definately depends on your niche as that determines how much you get per click. I suppose your question is sort of like, how many t-shirts do i need to sell to make $20 a day. It all depends on how much you make per t-shirt. If its $1 you need to sell 20. if its $0.01 you need to sell 2000. I hope you get my point (thought I would try and give an unrelated example!)
It all depends on a number of variables. Your target market and the specific keywords your site is optimized for, as well as the quality of your backlinks, will play an important roll. Every keyword has its own CPC. This prices varies depending on the forces of supply and demand. So it is not just a matter of how many visitors or how many clicks you need, but the quality of the clicks. The question is: How likely are those clickers going to make a purchase or produce an action for people and companies that are conducting a PPC campaign in Google. Of course, impressions and CTR are very relevant.
It is basically a combination of all posts above. Depending on your Niche, you may earn more per click or less. I would also say 1000 and more visitors should do it. Again, it all depends on your site's layout and whether visitors would 'want' to click the ads. I had a website with 3000 visitors per day. My Adsense was not placed correctly and I earned less that $0.50 per day. After fixing it, my earnings went up to about $3.
well it depends on your targeted words. If the Google CPC rate is good then you may earn this amount with little traffic but if CPC rate is lower than you will need more traffic to deserve this amount.