So long as the tuber is healthy and has the usual water, food, good light, most tubers have a number of growing points that develop into shoots.

They can also develop after one of those minor disasters where something has broken the first stems.

You might only get two shoots from a tuber but, if you're not growing for competition, this will still be enough to give a splash of that particular colour in your display.

I was told by White Flower Farm to allow no more than three buds to grow from the tuber.

That's good advice - I try to limit to two. If you allow five upright stalks to develop, it can get top heavy and break and the tuber has to work that much harder and won't develop. BTW - if you pinch out extra stalks at the base when they are 4" tall, you can pot them up as cuttings in the spring.

The exception to this rule is the hanging begonia which should be allowed to grow as many stalks as it can produce.