How many does it take?

Well, when my site was first created it had maybe 20 backlinks and got a PR of 5. It has now got over 1000 backlinks and has still got a PR of 5.

I don't think there is an average number of links.

Yeah, there really isn't a specific rule for x amount of sites. It depends on the relevent content, quality of those sites linking to you and other important elements.

If you were to do just one site; but that site was perfectly coded was huge and powerful and userfriendly and brought Search Engines and sucked in users whatever the Address because of the ease of use and power then you'd be chosen over somebody who had 500 quick sites to get money.