There are situations where 9 credits might make sense. Mine were in private colleges close in age. Discovered after couple of years if you took 11 credits rather than 12 considered part time. Based on pro rata cost saved 25k a year on tuition for one kid and they made up the 3 credits in summer.
Many colleges today have a specific dollar tuition for 9-18 hours/sem. Same no matter the number of hours. Makes sense to get in as many as you can.
Back in my day, the max limit was 21 hours so tried to get in 19-21 during the year and 7-9 in the summers. Still had plenty of time to work part time, be in extra curriculars, student gov., be in a band, date, go to every game and generally goof off. Best of times.