I also found it a little confusing initially when purchasing tickets last summer. Although we booked a private guide for the day she wanted us to purchase our own tickets in advance for specific times. In case this is useful for anyone else, from my notes:
This website helped me: https://theportablewife.com/travel/destinations/one-day-in-ravenna-itinerary/
It has a hot link to the ticket purchase page. Look for the text "purchasing advanced tickets online" on the page above. If purchasing tickets from this website they only become available about two months in advance. Like many Italian ticket websites, there is no indication that you're attempting to purchase too many days in advance; it just looks like there aren't any tickets available.
It's now required to book a time slot to visit the Baptistery of Neon and Mausoleum of Galla Placidia, I believe the ticket purchase webpage guides you through this requirement during booking, giving the chance to reserve your time slots for these two sites. The tickets will be emailed to you, which you can either print out or show on your phone. If I remember correctly, you can only book time slots for those sites; the other sites on the multi card can be visited any time on the same day.
The top UNESCO-listed sites in Ravenna may only be seen on combination tickets. The three-site pass includes the Basilica San Vitale, the Basilica of Sant’Apollinare Nuovo, and the Archbishop’s Chapel. The five-site pass adds the two sights that require time-slot reservations: the Mausoleum of Galla Placidia and the Neonian Baptistery. Single-admission tickets are not available for these five sites.
Have euro coins if visiting the Arian Baptistry, which is not included in the multi site ticket. Before you can enter, you’ll need to buy a paper ticket from the machine out front. The machine doesn’t take credit cards. I think it was a few Euros per person. When we visited on a Sunday afternoon a man was operating the ticket machine; We couldn't tell if he was 'official' or just looking for 'opportunities' but it worked out fine. I believe this site is only open Sunday and Monday.
The Domus of the Stone Carpets mosaics/Domus dei Tappeti di Pietra is also worth seeing, particularly to get a feel for the layers of archeology that exist under Ravenna.
The Basilica San Vitale and Mausoleum of Galla Placidia are next to each other, it is around a 15-minute walk to the Basilica of Sant’Apollinare Nuovo, and another 15 minutes to the Neonian Baptistery, which is next to the Archbishop’s Chapel, and about 15 minutes from San Vitale.
Going early and late to sites will help you avoid the day tripping crowds.