How much space junk (assuming it's magnetic metal) would it take to affect the magnetosphere?

Well... depends on how much you mean; they all do, a little. But, since they're in orbit, the effect is localized and always moving, so the net effect is zero.

Like, a whole nuther earth full

A few million tons, I expect.

Hard to tell, there is a LOT of stuff out there now! But, I can't imagine much of it is iron, seeing how heavy & magnetic it is, there are lighter metals designed for such use, so. Probably very little.

probably like the rings of saturn amount