15x15 is not very big. I would go with one plant, honestly. You could probably do two but as the plants grow and the weather warms up you will eventually be watering them every day, likely twice a day. As well as needing to fertilize even more.

Rodney

You can plant as many as you want in that container, BUT the plants that will grow will, taken together, not be a lot more than one plant in a spacious container. So you could plant one, to make it look like one full sized plant, or you can plant six, to make it look like one full sized plant. Recommended soil areas for plants are chosen to make the plant perform maximally. I often plant out two seedlings together, such that if one dies, the other will fill the space. But the two together will behave much like two stems on a single plant.

A 15x15, and you are also planning on diil and flowers in there? That's not much more than a 5 gallon bucket. One cucumber, or twin it if you want a pair together. You might be able to get away with 2-3 regular vining cukes with a good mildew resistance if they are spaced around the edges and also have plenty of trellis to space out on. But Patio Snacker is short, has most of it's foliage close too- and it can be prone to powdery mildew if not given space to breathe. Foliage can spread pretty plush.

I would also recommend using a mulch instead of flowering plants for soil cover. Your cukes will need as much as they can get in a container, and while putting in a couple short dill might not be so bad, also adding in flowers is asking a bit much out of your pile of dirt in there. I happen to like using last years fall leaves or grass clippings on containers, but you can use other mulches if you have them.