If you would like to grow green bean (bush type) in contains, start out with pots that are deep enough to allow the roots room to grow and still have drainage. You can put about 36 seeds in a 30 gallon bucket (which is about 30" x 20" x 17" in size). Some gardeners only feel comfortable putting 4 or 5 seeds in a pot the same size as this, so a bit of trial and error is needed, and the number of seeds will depend on the quality of the soil that you are using. The soil mix should be at least 8 inches deep.
You can also make a self watering container, by either using a drip line, or by having the bottom of the pot sit in a pond of water. Just make sure there is plenty of dirt between the roots and the pond. You do not want the roots to sit in the water 24 hours a day. Don't forget that adding compost to your soil will drain off into the water using this method and may cause a smell.
In standard gardening (instead of container gardening which is talked about above), green beans are generally planted 3 to 6 inches apart. Root crowding and competition between the plans is almost never a limiting factor for the bush and vine varieties. Typically access to sunlight is the biggest limiter on how many seeds you can plant. If the plants get too dense, the ones in the middle that become shaded will stop producing and eventually die.
One of the benefits of container gardening, is that you can rotate the container, so that there is more sun exposure for more plants, and this can not (obviously) be done with plants that are planted in a traditional garden.
Beans (and all legumes) are suppose to use atmospheric nitrogen when growing, and are thought to actually add nitrogen to the soil. Some gardeners dispute this, however. When using a container, however, often you will need to fertilize with a 24-8-16 or a 3-1-2 ratio fertilizer, if the plants are not producing beans.
We hope that you have found this helpful!