There is no exact answer to how many fish you can have in one fishbowl. The number depends on the size of the tank, and the species you want to have. The recommended ratio is that one small fish will need five gallons of water to properly live and swim. In order to properly calculate the surface area, you will need to measure both the length and width of the surface area (the place where the water and air meet). A bowl that will suit the entire family should be able to have one gallon of water in it. The bigger the bowl the better – bigger bowls will be able to fit more. Just make sure you get a bowl that is strong enough to withstand the water pressure and the fish. In case you want to have one goldfish, you can get a small bowl. If the sides of the bowl are straight up and down, you can just measure the length and width of the upper part. However, with a barrel-type bowl, you will need to measure the length and width of the bowl at the waterline. For circle areas, you will need to measure the length across and divide by two in order to get the proper radius, which you will need to square and multiply by pi or 3.14. It sounds like a lot of math, but it will need to be done if you want the fish to have a proper environment.