Hi Steve,
No there is no maximum number of assets you can import in your WordPress blog.
However, what may have happened is that your server blocks the script after a certain time, to protect itself.
Have you tried to re-run the script? If everything went well until now, the assets that have already been imported have a new path using your new blog URL and no longer use the TypePad URL. The script uses the TypePad URL of your previous blog to detect what are the assets to import (that’s why I ask for it at the beginning). Re-running the script may potentially work, because the script should only try to import the assets that have not been imported yet.
Let me know if this works.
Best regards,
Romain