The A320 MEL has a helpful OPS procedure (35-30-01D) to calculate the required oxygen, based on quantity per bottle and amount of bottles. If there isn't enough O2 on board the passenger numbers may have to be reduced. 750l O2 is enough for 193 passengers, so would cover the 189. However, we do carry 202l bottles, so that procedure wouldn't help much on 311l ones.