These filters are sold as single filters.
I believe it is a matter of taste and opinion. I've had my unit serviced by the same company for about 6 years now and not one tech has said a word. Until this year. He recommended using a filter without pleats. Claiming that the air flow would have less stress with a filter without pleats. Haven't tried it yet since I usually take advantage on the multi pack deals.
There is no expiration data on a pleated filters as long as they are kept clean (in the shipper carton).
The higher selling volume items typically have a shrink wrap. Your size id not very common and that my be why it was not wrapped.
You have purchased one of the most efficient filters sold in The Home Depot system. But in most HVAC systems there is enough fan capacity for enough air flow. That said you should check the filter every 30 days and change them at least every 90 days.
I do not know. I suggest you look on line or call the store.
1 filter. Always delayed every shipment. On auto-shipment, same week every 2 months. Why is it always delayed?
The initial pressure drop is roughly 0.2" w.g.