How many stems for tomatoes in containers?

[Merediana]

Coming from an "one stem only" background I was quite surprised that most people here seem to leave several stems or even don't prune at all. I've always been told that this doesn't work and you'll only get diseased plants.
So now that I've learned that this isn't true, I wonder how many stems you recommend for tomatoes in containers? And how much does this depend on the size of the container?

Where I live the recommended size for tomato containers is 3.3 gallons, here it seems more like 10 gallons which makes a huge difference...