That'd be enough for one on its own, so long as there isn't anything else in there. Also, bear in mind that axolotls can (and will!) get over 30cm and live 10-15 years. The first 3 I got 8-9 years ago are all over 30cm now! If you still think you could give an axolotl a home, they prefer shallow water if possible at an absolute minimum of roughly 40l per axie