Ravenna

You need at least a full day to see all the mosaics.

We spent 2 nights
There are 5 mosaic sites in town and another 2or3? outside of town

You could conceivably see all the sites in town during a long stopover but Ravenna really is worth an overnight

There are two days' worth of sights in Ravenna, if not more. In addition to the well-known mosaic sites, there are some other more minor ones and a mosaic museum. The local art museum normally has an exhibition of contemporary mosaics, but that section of the museum wasn't open last September; I think they were working on a new exhibition.

Ravenna also has an attractive historic center.

Two nights is just right.
Give yourself a full-day of exploring and checking out the various historical sites. I believe Ravanna is second, after either Rome or Florence for most UNESCO World Heritage Sites

Yes, do stay a night or two.
It's easy to walk around to all the mosaic sites, and the one out of town in Classe is a 15 minute local bus ride away.
It's a pretty town and really nice to wander around on a sunny day.

Any recommendations for hotels in Ravenna?

We stayed at Hotel Diana. It was very nice, not overly expensive, good but not exceptionally great breakfast.

We stayed 2 nights, and were satisfied that this gave us the time to see all the mosaic sites. Plus we went to a concert in a church.

We enjoyed Palazzo Bezzi, a modern 4-star hotel an easy walk from the train station, and just a block from one of the main mosaic sites.

Hotel prices in Ravenna are likely more expensive than when we stayed there for 89€. Cruise ship traffic will bring more people to Ravenna.