What is the mirror for installing updates? Why it is showing different no. of updates when i changed the URL in
/etc/apt/source.list to different URL ?
The main mirror is the "archive.ubuntu.com" url.
These own mirrors are run by individuals. That means, some of them are faster, some of them are slower. If you want the latest and greatest, with fast speed, use a different (larger) country's mirror. Like I live in Hungary, but our mirror is often outdated. So I just use the mirrors can be found in Germany, Switzerland, Austria.