For clarity to your your Description “Daily Every 15 minutes between 8AM and 8PM”
Your proposed solution of 0 0/15 8-20 ? * * * would be Every 15 minutes, between 08:00 AM and 08:59 PM
To fall within the original ask, you should drop the max hour down from 20 to 19 so it would be 0 */15 8-19 ? * * which is Every 15 minutes, between 08:00 AM and 07:59 PM
Otherwise you will get 4 more jobs than you might have expected each day.