You should read the code again and then this time better 1073741824 is only the theoretical maximum count, see the "break;" in the if-FileExists-else-branch.

Every training data set file is a single own game total round, and these files can by different human players from the developer team, so the training routine searchs all data set files in the input directory and then processes these one after the other many times (<= TeachIterations).

Did you understand it now?