I just heard from other webmasters that a link from one domain is count as one back link only. Is that true? For example I have footer back link from a site with 20,000 pages index by google does this mean that I only get one back link count from this site even do my link is in all 20,000 pages of the site with multiple pages index by google?
I believe you are talking about site-wide links. To answer your question "Sitewide links will not be considered as a single link nor would it account for all the links coming from on particular website". It is important to understand that each page will have its own authority and search engines consider all those links as your back links. Also they are not like having links from different web pages. If site-wide links for a major portion of your back links then it looks very spammy and may attract the wrath of search engines in the form of penalty. I advise against it. It doesn't look natural.