How bad does a shot in the mouth hurt?

nxt time i go to the dentist i get to fill my cavities ( three) and they said there gunna num me, so how bad does a shot hurt 1-10 and how long it is, and do they out like gelly on the area that nums in for a couple mins then give me a shot? im confused and scared! i hate shots1 i'd rather die then get one, help