If you are forced to grow dahlias in pots, the larger the pot the better. This way, there is more constant moisture in the soil at any given time so the roots don't dry out between watering. Lack of water will cause the plant to not reach its predetermined genetic height, and not be as full as they should be. Most full-sized dahlia plants IN THE GROUND are two to three feet wide, and 3.5-5.5 feet high when full grown.

If you put more plants in a small pot, you would be exacerbating an already stressful situation. Can you do it? Sure! But the blooms might suffer as a result.

It would be better to make sure you fertilize with a time-release tomato fertilizer, and water every day (once you see green growth) until you see it drain from the bottom. Also, pinch out the tip of the plant after it reaches 3-4 leaf sets so it becomes bushier instead of spindly.

Here's a couple images of full-grown dahlias to give you an idea of what size they should reach in optimal conditions:

These plants were not stopped after 3-4 sets, let go to bloom from the top down. Notice how they are not as full as they might otherwise be, with no blooms in the lower parts of the plants yet.

These were stopped early, and spaced too close together at 12 inches. Really, they should be 18-24 inches apart, with the giants at 28-36 inches apart. I would get twice as many blooms if I had another bed and spaced the plants more, but quantity of blooms per plant is not my focus.

When I did one plant per bucket they did get about that tall, but I didn't pinch them off so the space looked sparce so I was thinking of putting 2 in a bucket this year. I do have a slow release bulb fertilizer that I used and it seemed to help keep them happy.

So, is it the water that's more of a concern or the space? I can make sure they don't dry up. I live very close to where I work, so I could water them before work, at lunch and then again after work if I had to. I have seen online where people have planted 3 or 4 dahlias around one steak, but that was in the ground. If its water that's the bigger issue, I can take care of that, if its space, I can just plant one tuber per bucket. What do you think?

More the watering and fertilizer, as both I and other growers have sucessfully grown full-sized plants in four inch containers planted complete in the ground. It results in a small clump of tubers.

My experience with planting in pots outside the earth was half the size with quarter the number of blooms, so I don't grow in pots anymore. But I am fortunate in that I also have the ability to plant in-ground as I please.