Documentation says that

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There’s no chance that the reference count can overflow; at least as many bits 
are used to hold the reference count as there are distinct memory locations in 
virtual memory (assuming sizeof(Py_ssize_t) >= sizeof(void*)). Thus, the 
reference count increment is a simple operation.
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But Py_ssize_t is a signed type so in case of sizeof(Py_ssize_t) == sizeof
(void*)) it can only reference a half of memmory :). But because any python 
object consumes much more than 2bytes this is not a problem.