yup I would do it like this as well. In this situation of something spawning in the volume do a check at spawn to manually set variables. In my situation I have players spawn inside a volume like so and I will tell the volume how many players have entered. You should also make sure that these variables are capped to never go past your max number of players and never go below 0.
This volume also triggers other spawning actors for zombies which I toggle active or inactive based on these variables. I use dungeon architect to procedurally spawn cities with also procedurally place zombie spawners. At level load I could do an initial sweep to check for spawners or I could do that way before while the player is loading in and keep track of all the assignments in a save file.
The reason this is difficult is because it’s blueprint. Blueprint is supposed to be like this because it forces to learn C++ which is for the most part better for everything. Don’t get mad, go learn 