Git commits do not have an index number. A git repository is a tree of commits, it is not a list.
git log can order commits in the following ways:
Commit Ordering
By default, the commits are shown in reverse chronological order.
--topo-order
This option makes them appear in topological order (i.e. descendant commits are shown before their parents).
--date-order
This option is similar to --topo-order in the sense that no parent comes before all of its children, but otherwise things are still ordered in the commit timestamp order.
--reverse
Output the commits in reverse order. Cannot be combined with --walk-reflogs.
If git log would show you an index number it would change each time you show the commits in a different order (and if you merged in another branch).