Could existing ventilators be modified to increase the availability of ventilation?
This paper “A Single Ventilator for Multiple Simulated Patients to Meet Disaster Surge” suggests that four people could use a single ventilator in a disaster situation
This blog suggests that the number of ventilators would need to be doubled to cater for a pandemic would be an extra 200,000 in the US. This would triple the current number. The paper above suggests it might be possible for current ventilators to cover this shortfall.
All these ventilators need to be manned by skilled personnel. These medical staff will themselves be affected by the flu pandemic. So the numbers of skilled staff to operate and repair ventilators and to nurse pandemic patients is still a constraint.