Since there is just one hot tub, there would only be one instance.
There is nothing in your class that forces it to be a singleton. If you're not familiar with that term, you need to become familiar with it.
In the doorbell/butler analogy, you are assuming that only one person will live in the house, so you are assuming that the butler will know to tell the one resident of the house about events involving the doorbell.
As you can imagine, those assumptions are invalid.
Now, you could have the class keep track (the constructor is a good place to do this) of the last instance of the class created, like so:
static someClass *pInst;
static void CallbackFunction();
someClass *someClass::pInst = NULL;
pInst = this;
Register someClass::CallbackFunction() as the function to be called when the interrupt happens.
From this, it's relatively easy to see how pInst could be replaced with an array of instances, and how, in the constructor, the new instances can be added to the array, and how, in the callback function, the callback methods for all the instances can be called.
If you are happy with assuming that the class will only be instanced once (per Arduino), then it isn't necessary to worry about multiple instances.