The V$SGA_TARGET_ADVICE view provides information that helps us in deciding optimal value for SGA_TARGET. MMON background process gather statistics about sga_target usage and update the V$SGA_TARGET_ADVICE view.
Check SGA Target Advisory is enable?
a. DB_CACHE_ADVICE should be ON
SQL> show parameter db_cache_advice
STATISTICS_LEVEL should be TYPICAL/ALL.
SQL> show parameter statistics_level
SGA ADVISORY REPORT:
SQL> select * from v$sga_target_advice order by sga_size;