You would first figure the total surface area of the water garden. Multiple length by width to find the area surface coverage. This will give you an idea as to how much oxygen will be available to them. If the surface of your pond has some coverage of floating plants you would include this area but don't include areas that contain marginal plants. Each inch of the fish should have 6 square inches to 1 square foot of water. Koi, however, need more space, and should have at least 25 square feet for every fish. You want to always go on the side of too much space rather than too little. You can use the following table to help you stock your pond.
1-2 inch fish - 1 square ft. 3-4 inch - 2 square ft. 5-6 inch - 3 square ft. 7-8 inch - 4 square ft. and so forth.
If you have aeration in the form of a waterfall or fountain, you can add a few more fish. A waterfall will actually aerate the water substantially and you could double the number of fish. Where as a water fountain doesn't provide as much aeration.