There isn’t one, and I don’t think there will ever be one. Most folks making a first post as a thread are trying to find help; even if there’s a thread on that topic on the portal they might not know to look at it until AFTER their first thread.
But the bigger reason for not having a “X posts to start thread” is this: They’ll (both spammers and non-spammers) just post all over OTHER threads looking for whatever they’re looking for, often wildly offtopic–and the spammers do that already. So instead of seeing new thread with a lot of links pop up, you’ll see an older thread pop up and think that “Oh, someone just posted, let’s see what’s up–Rats, spam” after opening the thread.
There is an “X posts to Y” rule, but it’s 50 posts and positive reputation to be able to give non-neutral reputation.