Proline boxes are 10 units for several reasons: to have a more economical price and help starting tattoo artists to buy several models and test until they find the models that suit them. Another reason is to avoid forcing tattoo artists to buy 20 units of some models that they only use sporadically due to their size or gauge, thus avoiding an investment in something that in the end they might not end up using because it is not the model they usually use.