Three short stories or one novel for a full membership, one short story for an associate membership, the last I heard. But your sales have to be to markets that pay professional rates. As Bookstop says, the specifics are at their website.
I used to belong, but I let my membership lapse several years ago when I stopped writing for a while and sought gainful employment instead. The biggest benefit, for me, was receiving their publications, but you can subscribe to the Bulletin (which is the most useful, because it includes great marketing information), without a membership.