How many of each rare item?

None of the gen 3 Pokemon will use these items. If you wanna look very far into the future, aka gen 4, if I remember correctly they won’t use these items either.

I keep about 5 of each, except for the Metalcote which I keep 10 of because that can be used on Scyther or Onix.

5 metal coat and 3-4 of the others.

I keep no more than 4 of each for two possibilities: I get a perfect mon that currently uses them to evolve or Niantic decides to deviate from the main games and reuse them for future gens.

Enough to make two of each mon that could need one. So 4 Sun stone, metal coat, and king's rock, and 2 dragon scale and upgrade.

I keep just 1 of each, why do you need more?
Except in the case of Sun Stone because that yellow seed pokemon will only evolve through using one.

I was keeping 5 of each but I've dropped down to 2 or 3 depending on the item.

I was blessed with not only a 100% oddish, but a Sunkern too. Maxed them both out. Not really sure why I keep any sun stones anymore...

Keeping 5 of each except dragon scale which I am saving for evolving Kingdra for dragon medal.

I keep none. They're not worth the bag space and there's no real immediate need for them as none of the Pokemon that require them are very good. I used to keep Sun Stones for evolving Sunkern as I always found them aplenty, but they have become quite rare here due to the weather.