The below updated parameter value has been set currently in postgresql.conf file.
autovacuum_analyze_threshold and autovacuum_vacuum_threshold both parameter values set from 50 to 20.
Created one table with 100 records. First, we deleted 21 records. after an autovacuum does not happen automatically. After again I deleted another 20. Out of 100 we delectated 40 records then the autovacuum happened automatically. As per the configuration Autocacuum will automatically delete 21 records in the table but my case does not happen.
If you want vacuuming by autovacuum to be driven entirely by autovacuum_vacuum_threshold, then you should set autovacuum_vacuum_scale_factor to zero.