How many pages are needed to get top 20 rank?

change the question to "how many links are needed to get top 20 rank?"

 

That's like asking how much money does it take to buy a house?

There is no correct answer without details. For a non-competive keyword, you might only need 1.

Every single keyword has a different answer. The best thing to do is to do a search on the keyword and look how many backlinks the sites that are in the top 20 have. Even that won't answer the question, because you need to know what the PR and relevance of the backlinks are. Backlinks are only one part of any search engines algorithm. Content, domain name, page names, title, etc. are all other factors. The number #1 site for hosting only has 441 backlinks in google. The number #2 site has 3,110 #4 has 77,700.

Some highly competive keywords have top ranking sites that have PR9 or PR10 links - or use cloaking and other blackhat SEO. There is a huge difference between a top 20 ranking for "sex" or "hosting" vs. "typewriter repair".

It's a question that cannot be answered.

 

I've not seen that to be the case.

 

A simple example. Say I was trying to promote "Florida Vacation". One site have only 2 pages covering Orlando and Miami. Another site has 300 pages showing 300 different cities in Florida.

Given everything else being equal, SE will rank the site with the 300 pages higher than the site with only 2 pages if the search term is "Florida Vacation", because it has more content for "Florida vacation". Of course it may not be the case if the search term is just "Miami Vacation".

So it all depends on the search keyword and the content of the pages. That's why I said "everything being equal", in general, larger sites with more pages (more content) will be better SERP.