Well. technically they could, I suppose it would depend if they even get along. If you let them you could end up with a ton of fry, which you most likely would not have somewhere to put. Especially with goldfish, even the fry need a lot of space. If they do breed, they'll end up eating the eggs anyway, if you let them.
Hard to say exactly. It could be 1.000, could be 3,000. It could be even more. Really it depends on how many survive, and how many were actually fertilized. You also would have to cull some of them, if they have bent spines or other issues. It can be difficult to know how many fry you will actually end up with.