In September of 2019 we spent 4 days in Rome. We stayed near the Spanish Steps (a delight in themselves). As part of our exploring of Rome we walked past Trevi Fountain 6 times. All different times of the day and night. It's amazing to see how the lighting from the son in different positions throughout the day can change the experience. Everytime we walked by we stopped to watch the fountain, and the crowds.
If you visit during the day or evening the crowds are huge. I highly recommend going early in the morning. One of our visits was at 6:30am and there were only about 40 people around the fountain so we could get our pictures and toss a coin in.