These are the blades you want if you're buying an injector razor, like the ones made by Schick, Personna and Pal. These American-made, PTFE coated blades come twenty in a pack, and I find they last ten to fifteen shaves each. That means this single pack of twenty blades could last nearly a year, maybe more if you don't shave every day.
PTFE-- essentially the generic name of Teflon-- protects the edge before its first use and the bit that remains contributes to the blade gliding easily over your skin.
These blades are sometimes referred to as "Ted Pellas," after a lab supply company that used to be one of the only sources of them.