I wonder whether that counter could overflow.
It seems like Cython would raise an OverflowError when converting a too-long python integer to a 32 bit (C) int, so at least we would notice.
The smallest chunk attic can create seems to be 1kiB. So it seems the worst (and rather unlikely) case is an overflow if you have 2 TiB data made from 2^31 repetitions of that 1kiB chunk.
Is there any frequent chunk so this could be a problem in practice for multi-TB backups?