I have bought Dahlia tubers from a big box hardware store. According to the packages there are 3 plants per package.
Each package contained 1 clump that was about fist sized and two smaller clumps (size of the smaller clumps varied wildly).
I bought 3 packages: Blue Boy, Thomas Eddison and Arabian Night.
I have some pots that are 30 cm wide and 25 cm high (around 12 inches wide and 10 inches high in the imperial system).
I've read that I should plant these indoors 4-6 weeks before the last frost date which is about now.
Considering I want to maximize the number of tubers per pot, how many per pot can I realistically plant.