Quest system help pls pls add

Hi I really need help with a quest system pls!! I’m trying to make a entity which is Naruto, that gives you a quest to fight 10 ninjas. BUT I’ve been trying to see how to do this but i can’t seem to figure it out. Maybe making a variable to when I kill 10 ninjas I get quest complete and a new variable is set to true which opens up a new gui. BUT HOW DO I CHECK FOR HOW MANY TIMES I KILLED THE NINJAS?? HELP PLEASE