Hi
Welcome to the forum
Rules only fire once their condition is satisfied, and not any more, which is useful in some circumstances but not for your particular use. You are better setting the default value of your count field to Count() and leave the "Update this value when the result of the formula is recalculated" ticked, that way it will add and subract to your hearts content.
Double-click the count field and click the fx button and add the function count() and insert one of the fields you wish to count, job done.
Best of luck with this wonderful tool
Cheers