what is the mortality rate of the baby goldfish?

Well, that depends on a few things, really. If you have never raised goldfish before, its more likely you lose a few more fish than you would if you'd done it before. Its just a natural thing of experience, which only comes with practise.
If you can spot the first signs of fungus, and add the appropriate meds into the fry tank when they are still eggs, you raise your surivor rate quite good. Fungus develops mostly on unfertilized eggs, and spreads to the fertilized ones, and a great deal of fry might die. Experienced breeders put methylene blue into the egg tank right from the start, and prevent that kind of thing.
Then there is feedings, water quality, technique when changing water, and water volume. All those play a role in mortality, even the depth of the water. The deeper it is, the more pressure is in the water, and those sensitive little things gets their spines crushed, and it doesn't need much force to do so. A little too much current when changing water causes the same thing.
All in all, if you'd do everything right, handled them carefully, take care of the water quality, feed them nice quality food a few times a day, the mortality rate shouldn't be too high.
There is still the question of culling though, which also lowers the number of fish. And depending on what you are trying to accomplish with the breeding, the culling brings down the number drastically. For example, you are trying to breed ranchu, a lot of the fry will have dorsal fins, undesirable tales, crooked spines (will happen with any breed, btw. ) and other deformities. You can decide to humanely dispose of the undesirable fish, or raise them separately, trying to find homes for them with people who do not care if their little fish is less than perfect. That is basically what most pet store fish are anyway - culls from breeders. They make loveable pets either way...

I don't really mind having deformed fish.i already have one in the main fishtank ...