Each car needs a separate policy, but if you get all your policies through one company they are likely to throw a discount at you for multiple cars. OTOH, it also means if you get in an accident or get a pile of tickets your rates will probably go up on all of them. And, of course, you probably don’t need collision on a shitbox car that’s paid off.
It may also make a difference what you use them for. They may give you a lower rate if they know it is not used daily. I know someone with an old 60’s-era Mustang, he has it on a special policy that is much cheaper because it’s for classic cars only. But there is a limit to how much he can drive it in a year before his rates go up. It’s basically for old cars that you only drive to car shows or just to show off, and keep off the road entirely in the winter.