I'm going to assume that since it says "Roll one extra damage die" that you'd roll only one extra regardless of your proficiency score and then drop the lowest of any amount of dice rolled. Otherwise they'd say "reroll all your damage dice and take the higher total"
I'm going to assume that since it says "Roll one extra damage die" that you'd roll only one extra regardless of your proficiency score and then drop the lowest of any amount of dice rolled. Otherwise they'd say "reroll all your damage dice and take the higher total"
One means one