Can you be more specific in your question? Do you want to buy a colony of 15 ants (14 workers+1 queen), or do you want to buy 15 colonies (and thus have 15 queens). If it is the first, you just need one box (I assume you mean the area where the ants forage with the word 'box'), if it is the latter, you need a seperate one for each colony. But why would you want 15 colonies of Lasius niger?
One thing is sure: if you have multiple colonies, you need to keep them separate, or they will fight till only one remains.