How do I change a variable that is in a different blueprint?

You have to get every actor from the array via an For each loop
plug the array in the left side and on the right you get an actor and an integer. you probably just need the actor reference to plug into the cast to node and that into the set max zombie node like above, but make sure that if you want to add something behind, you have to get it from the “Complete” execution on the for each loop otherwise it will integrate it in the looped actions.