You can use 3 small tubers spaced evenly around the edge. If 3 tubers appear too much, you can use 2. I would definitely use more than 1 in a 12" basket.

You need to water religiously or they dry out. fertilize regularily with a mild flowering formular throughout the season. Although they need regular watering, they hate wet (saturated) conditions, so use a good container potting mix (well draining) instead of just regular potting soil.

For better results and earlier bloom, start the tubers indoors in regular small pots and when they are 3-4" tall, transplant them to the baset. Initially they grow "upright" but once they get larger, and flowers start, they will tip over and start to hang. It is actually the flower stalk which elongates (not the plant itself) and it is the weight of the flower that pulls it over the side of the basket. If you have grow lights, this is preferred, otherwise a window will do.