As noted in Misc/SpecialBuilds:

COUNT_ALLOCS
------------

Each type object grows three new members:

    /* Number of times an object of this type was allocated. */
    int tp_allocs;

    /* Number of times an object of this type was deallocated. */
    int tp_frees;

    /* Highwater mark:  the maximum value of tp_allocs - tp_frees so
     * far; or, IOW, the largest number of objects of this type alive at
     * the same time.
     */
    int tp_maxalloc;

...
We use this for Fedora's python debug build to get some interesting debugging info. (If you try to "grep -r" through Python 3.4 source code, you'll find quite a few ifdefs with COUNT_ALLOCS.)