They do fine in ten gallons of water if kept singly. Many folks suggest 20 gallon tanks for the fact that once you add a substrate, hides, plants, filtration and all that other stuff, you have about ten actual gallons of water left in the tank.
This is where a calculator and those darned formulas were were supposed to learn in school come in handy. Figure the volume of the tank empty (LxWxH). Then figure the volume of the substrate, if any by estimating it with the tank dimensons, substituting the depth of your substrate for the "H". Subtract that from your tank volume. Plants and hides are a little harder, so it is best to estimate them by taking a pitcher that has graduations on it and filling it up with water. Then add all your plants and tank decorations to it. Catch the water that overflows out in a pan. Measure the volume (more math to calculate this!) of that water and subtract it from your previous results from the tank and the substrate.
If you end up with ten gallons of water volume or more, you are good to go for one axolotl.