Just thinking @Paul Anthony maybe your begonia corms are those very small ones? If they are about 1 - 2” across, you could put more in your containers, 2 or 3. All depends on the size.Our GC has some huge ones, about 3 - 4” across, those I would plant singularly.
You really need to know what the bulb is before you plant it, some bulbs you leave the pointy bit stuck out of the compost. Well done on starting off your begonia corms, keep them in the warm, perhaps you can put a photo on when they start to shoot and someone can give you more instructions from there.
Perennial gardener said:
I've seen begonia plug plants for sale online but curious if they are the same? Would they flower in the first year and produce a corm too?