Indoor Cactus/Succulent Garden
(Nixa, MO, USA)
Very full succulent planter
How many cactus & succulents could I safely plant in a container that measures 15"? The container is round.
Oh, boy, here's a loaded question! Without knowing more about where these will be displayed, and how big they are, there is really no definitive answer. It also depends on what kinds of plants these are. I would say that it's safe to plant most succulents much closer together than other kinds of plants.
As succulents tend to be so compact, and to have quite shallow root systems, they don't require as much room around them. The biggest issue and requirement with all cacti and succulents is that of the appropriate light. If your display is under a grow light, then you will be fine with more plants. If you're trying to have a window garden in a northern climate, then more room around each plant will allow more light to get to each plant. You can also add some pebbles, driftwood or other embellishments to make your container seem more established.
So, for actual measurements, I would plant smaller 2-3" potted succulents around 3-4" apart, and keep in mind their growth habits too. Trailing types can be planted near the edges of the container to flow over and taller more upright growing plants should be in the center (or the back) of the container.
The depth of the container will also make a difference in how closely you can plant them. Deeper pots will hold moisture longer, and also make the plants more stable. Shallower ones will dry out faster, and the plants may tend to flop over more.
The pictures above show some great examples of closely planted succulent planters.
Hope this helps to answer your question!