ResponsiveVoice will try to speak a very big number in its entirety, which in words surpasses the limits of the TTS engine.
As an example, the number
777,777,777,777,777,777,777,777,777,777
would be read as
seven hundred and seventy seven octillion seven hundred and seventy seven septillion seven hundred and seventy seven sextillion seven hundred and seventy seven quintillion seven hundred and seventy seven quadrillion seven hundred and seventy seven trillion seven hundred and seventy seven billion seven hundred ans seventy seven million seven hundred and seventy seven thousand seven hundred and seventy seven
A common workaround is to simply write the number out, instead of using its numerical form, as the TTS engine will interpret it correctly.