Summary !!

By default, SQL Server updates index statistics automatically. Frequency of automatic updates depends on number of rows in the table. If you allow SQL Server to update statistics automatically it will use the following rules:

1. If a table has 6 or fewer rows, statistics will be updated after 6 changes

2. If a table has 500 or fewer rows, statistics will be updated after 500 changes

3. If a table has more than 500 rows, statistics will be updated after 20% of the total rows plus 500 rows are changed (INSERTED, UPDATED or DELETED)

4. SQL Server uses the rowmodctr column of the sysindexes table to determine the number of changes since the last update of statistics