I have liability only, none of them are worth insuring for loss. Multi-car discount, and a couple of them I don't drive in the winter so I suspend the insurance for those.
Everyone's driving record and location is different, but on my three paid-for used cars liability-only is less than comprehensive on one cheap new car.
It helps that I haven't had a claim in 30 years.
How’s the registration cost? Where do you store them? How much does it cost/how much time do you spend keeping up with all of the regular maintenance?
Registration is minimal because the largest component of it is based on the value of the car. Don't ask me why that makes sense but that's how we do it. I do the maintenance myself as much as I can, it's a hobby that I enjoy (which I realize would not be the case for others).