I checked the number of my backlinks using an online tool. First day the number was 384. The next day when I checked it was 163. Again on the third day it showed 384. What could be the possible reason for this?
Seems more than likely that the search engine was recalculating things and the low number was when it was still incrementing (adding up the worthwhile backlinks it offers as a sample of what it has discovered, for owner or competitor analysis).
Honestly speaking, there are no accurate tool for calculating backlinks online for every engines using different algorithm. They are only for reference purpose, IMO. So, such fluctuation would be a normal issue. :)
I agree with Airforce One, the number of links you have tends to be entirely dependant on which "tool" you are using to check. Unless you have been keeping track manually (and let me tell you from experience that is a nightmare) you are probably never going to know exactly how many back links you have. I would say that in backlink strategies quality far outweighs quantity so try not to get bogged down by the numbers.
Constantly Google is updating backlinks periodically. However, just before they do a PR update, chances is the No. of links will move up and down pretty fast in those few days. I would recommend you stick to a more stable tool to check as well, such as SEO Firefox or SEO Quake.