I'm not sure how to answer this. You will need four numbers to calculate it: (1) the goldfish' oxygen requirement, (2) the volume of the goldfishs oral cavity, (3) the solubility of oxygen in water at the temperature you are using, and (4) the efficiency of a goldfish's oxygen intake.

The first number you could estimate as I described in your previous question. The second number you could estimate by looking carefully at your goldfish and measuring it with a ruler. The third number you could look up in a good chemistry book. I can't tell you where you could get the fourth, although some kind of college-level ichthyology textbook would be where I would start.