The amount of money needed to retire differs from person to person.
You can know how much money you will need to retire by -
calculating how much your expenses will be once you retire
your post retirement plans like traveling around the world or taking up a sport, a hobby etc
the number of dependents on you whom you need to support financially
unforeseen expenses that can crop up in old age
The money you need to retire is a corpus that takes care of regular needs plus unforeseen expenses.