I am checking regularly links for my sites on Google. Its my observation that the number of links shown are always fluctuating. Fluctuation is about 15% at times. Naturally the position in the rankings fluctuate accordingly. Could anybody explain why this should happens?
A few things all of which are out of your control. The quantity and quality of the backlinks to your site's pages. The quantity and quality of the backlinks to your competitors' pages. The relevance and type of content on your competitors' pages. You lost a new content "bonus" on your pages and now the search engines know how to better (and more accurately) rank your site. There are other factors, ones that you can control - such as the quality and relevance of the content on your own pages, how well they're linked to each other, and how they're structured. but those should be taken care of before the site even launches. But whatever you do, don't think you're being penalized or "sandboxed" - this is actually normal behavior.
While there certainly is a correlation between the number of links and the position of your pages on SERPs, it is not a one-to-one ratio or not even close. As Dan Schulz said, the position on the SERP depends much on the content on the page, the content on the competitors' page and the relevant inbound links you and your competitors are getting to your respective pages. For a more accurate link count on google, sign up for google's webmaster tool, where you can see the number of links you have, as well as the number of pages indexed by google (and some other good-to-know info).