Start with the number zero, and each time an object is created, add one.
That's the logic.
If you mean something like creating a class, and counting how many times an object of that class is created, put the functionality to do the counting inside the constructor and make the counter shared across all instances (for example, by making it static).
Don't forget that you will need to add the counter increment to the copy constructor and the assignment operator. You probobly want to have that as a seperate counter since those objects will be implicitly created.