Keep the same numbers as before- 20 advertisers paying an average of $.50/click. Let's say you have a page that display 5 ads. From that page, you generate an average of 100 clicks/day. So you make $50/day.
Fast forward- now there are 40 advertisers paying an average of $.40/click. You still have 1 page that displays 5 ads. So chances are you are still going to generate 100 clicks/day. But now you are averaging $40/day.
Why? Because you're still showing 5 ads/page. Instead of 20 advertisers competing for those 5 ads, you have 40 advertisers competing for the same 5 ads. But still, only 5 ads are showing on that page. The number of ads doesn't change even if there are 50 advertisers, or 500, or 5,000.