The recommendation to shelter-in-place using duct tape and plastic sheeting will be provided by local government officials only when an emergency occurs. The Red Cross recommends that people have these supplies on hand in case they are needed.
You would seal only one room when advised to do so, and do it only when instructed - not in advance. It is likely that one roll of duct tape will be adequate. Plastic sheeting of durable thickness (thicker than food wrap) is recommended for covering vents and other openings to the outside - not the entire room. It is intended to provide a barrier to air flow.
While we can not guarantee that plastic sheeting over air vents will stop all biological, chemical, or radiological agents, it will add to the barrier of protection for your safety.