So, the Today sponge
form of contraception was pulled off the market in 1994. Apparently Elaine
gets wind of this and finds a place that has cases
of them in stock, and buys them out.
So, now, she's got this closet full of sponges, but even though there's a lot there, there's only a finite number of them, and she's got a steady stream of boyfriends coming in, and now her decision on whether or not to have sex with them is contingent on whether or not they're worth using a sponge up for.
(this is a Seinfeld reference. If you didn't know that, you were likely living in a cave for the past 9 years.)