This one variable tile actually aggregates all of my leak, motion, and contact sensors’ battery health. (You can add multiple triggers in a rule if this was what made you think you had to do this for each individual device?) So the short answer is no, you don’t need to create a variable and a rule for a each individual device. But, if you want, you can create this aggregation for each type of device if that makes more sense to you. Ex: have one aggregation for all motion sensors, and another aggregation for all leak sensors.
Here is the quick example of my aggregation rule, and hope this helps:
Trigger: when the sink leak sensor battery is changed and more than 0, or when the living room motion sensor battery is changed and more than 0, or when the front door contact sensor battery is changed and more than 0
Flow: If sink leak sensor, or living room motion sensor or the front door contact sensor’s battery is below 80, set variable
$isAnyBatteryLow to true; else set
$isAnyBatteryLow to false.
And $isAnyBatteryLow is the variable I added in my dashboard as the variable tile.