1 each -242 O-ring. Doesn't hurt to have a spare around, but probably just one. See editorial comment below:
A word about O-rings: they have a shelf life of 8-10 years. Unless you're doing a lot of processing, they'll probably dry out and start cracking before you'll actually need them. (I've replaced two O-rings in the last three years.)
They're also an off-the-shelf item; most industrial supply houses carry them, so you don't have to worry about them becoming extinct.