It has very, very little to do with the number of pages indexed. Getting a page indexed and getting it to rank are entirely different.

One page wonders can get more than that. 1000 page sites can get only a handful of visits a day.

It depends also on whether the traffic is coming from the organic SERP's, or links from around the Web. If it's coming from the SE's then the pages need to rank for something. If it's coming from links around the Web, those links simply need be visible and of some interest to traffic on those other sites' pages. Etc.