Hi RJ, these are sold individually.
3/8”. It mentions on the packaging in store but not on ad online.
I'm not sure of the type of metal, but I've had this caster at the bottom of a dock gangway for over 3 years. It is exposed to the elements and constantly being used due to the tides, and has held up very well.
Sorry, I no longer have access to these casters. They are on a dolly in a church. I suggest you go to a Home Depot store and measure the mounting plate yourself.
According to the specifications, the wheel itself is 3" in diameter. The height of the entire assembly (plate height) is 3.75".
You could add a 0.125 inch (1/8-inch) shim plate on the shorter casters to bring the heights all up to 3.75 inches.
I don't think they will scratch, hard rubber.
The price is for only one caster. There is no package of more than one caster.