It’s important to understand that every page on the web with a link to your site counts as a backlink. Many people assume that a link from a domain counts and 1 backlink even if it is on every page. Also, every page of your own site that has a link to your domain is counted as a backlink. As an example, our site has about 2,500 pages, each with a link to our root domain, so that’s reported as 2,500 backlinks. I have over 700 posts on this forum, each with a link in my signature, now there are 3,200 backlinks. Then, if you exchange sitewide links with other sites that have thousands of pages, you get thousands of more backlinks.
Yahoo currently reports 5,841 “inlinks” to our site. Yahoo is the most accurate of the search engines, but just last month we had well over 10,000 links reported, so who knows what they have changed in their reporting. They do have a whole new site called Site Explorer that recently launched, so it probably has something to do with that.
Finally, it’s not the number of links that you gain for your site, rather the quality of those links that ultimately helps you in the SERPs. In other words, don’t waste your money buying 50,000 links from a ton of unrelated sites as you will likely be banned from Google anyway.