docs: def numKeys(self) -> int:
          """
          This will iterate only over the last chunk since the name of the last
          chunk indicates how many lines in total exist in all other chunks
          """
          chunks = self._listChunks()
          num_chunks = len(chunks)
          if num_chunks == 0:
              return 0
          count = (num_chunks-1)*self.chunkSize
          last_chunk = self._openChunk(chunks[-1])
          count += sum(1 for _ in last_chunk._lines())
          last_chunk.close()
          return count