Besides trying to predict the number of no shows to still have the flight full, consider that in some cases, overselling even beyond that may be profitable.

Example:
A hypothetical domestic flight might have an advance fare of $300 RT ($150 OW) when purchased well in advance. That same flight might have a last-minute walk-up price of $700 one way. If the flight is full and the airline anticipates no no-shows, they can still sell that last minute traveller a ticket for $700, then offer a $250 VDB voucher to get someone to volunteer for a later flight. Net profit from the oversell = $450.