Dá o bônus total, não o bônus adicional por nível.
In other words, suppose I level up to 6. Is my proficiency bonus 3 or 14?
São 3
Página 57 do documento de referência do sistema 5.1 dá um exemplo:
Your proficiency bonus is always based on your total character level, as shown in the Character Advancement table, not your level in a particular class. For example, if you are a fighter 3/rogue 2, you have the proficiency bonus of a 5th-level character, which is +3.