A trait is a phrase, not a number. "Steady under a shouting match" is a trait. So is "cannot leave a locked door alone", and the second one will cost you more than the first.
Traits are read, not rolled
Nothing adds a trait to a total. A trait changes which options a scene offers you, and a trait that never closes an option off is not doing any work.
How many
Start with three and let play give you the rest. A character who arrives fully described has nowhere to go.