You won't know until you get on the bus, how many people there are. It is a full-sized coach.
We just did one and I have one piece of advice - book it with the land tour first. The land tour is great, but frenetic - new hotels, lots of excursions, a couple afternoons of downtime. The cruise is much more relaxing. If we did it again, I'd start in Fairbanks and take the tour first.