It depends on how technologically advanced the society is and how sociologically advanced the people in question are.

Can 8 billion cavemen sustain modern civilization? No, they are cavemen.

How about a society with current technology where near everyone achieves doctorate level education, crime and other social problem are near non-existent etc? I recon low millions could keep going better than fine.

Provide sufficiently advanced society with sufficiently advanced technology and I don't think there really is a practical lower limit.