How much traffic can a site get from SEO?

Hello,  It really depends on a lot of things. Without knowing more about what market you are focusing on there is no way to answer. We would be more then happy to help you with your internet marketing needs or answer any questions you may have.
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Evermond Performance Marketing

It depends on your Search engine ranking . If your competitive keywords are well rank in search engine specially in Google then you have chance to derive more traffics because generally traffics comes from the search engines. So focus on your keywords and SEO practice. 

There is no quantitative answer to your question, but I can say that with the right SEO strategies and tools, you'd be able to maximize the potential of your site so that you would get to where you wanna be--on the top of the search engines ranking page, highly visible.

I got the idea for this answer in the articles of OIT2P , a BPO service provider .

The traffic will depend on how you build your website using backlinks.  Some high PR blogs had high amount of traffic because of reciprocal linkbuilding and also posting their links on high PR websites as well.