How many links does it take to get to the next PR level?

I don't know - most sites DON'T actually try to get listed in free directories - because they don't even know about free directories. PR0 links are actually pretty hard to hunt down, because any serious site that isn't just starting has a PR of at least 3 (that I've seen - I don't usually go PR hunting, but have seen that it is pretty easy to get a business site up to pr3 or even 4 with just a few quality links)