A gravel mulch helps to conserve moisture, and grouping the pots reduces evaporation, but container plants get through a lot of water. If this is a problem investing in an automatic watering system which will keep the plants constantly moist. Alternatively, grow plants that will stand some drought. I have a lovely pot of Aloe aristata, a succulent just about hardy with me. It will take days without water.
Containers look better in a group.
Unless your container is exceptionally decorative a group of containers makes a far better feature than a single pot. Pots can be added and replaced as required, the group refreshed as plants go over, and new combinations tried when you get tired of seeing the same arrangement. You can easily make changes in a single container, replacing plants two, three or more times a year. Try bulbs, wallflowers and pansies in spring, summer bedding for the hotter months, autumn and winter interest from chrysanthemums and foliage plants. Grow permanent plants in their own container but combine single season plants.