The solution I found in this topic: https://premium.wpmudev.org/forums/topic/formatting-of-multiple-choice-fields?replies=9#post-1185163, is perfect!
But I need one more thing in this css code. I need a condition.
If I have from 1 to 6 choices, I don't want the choices to split into 2 columns.
But if I have 12 choices, yes.
The solution I receive, in this same topic, is not what I want (or I didn't know how to use it with the previous code)
Thank you in advannce for your answer