In fact, as of just 5 days ago, the Marketplace hit the 18,000 app milestone, and has added over 1,500 new apps as of this writing to reach 19,637 total apps. In just the last, 24 hours the Marketplace has added 901 new apps, and looks poised to hit 20,000 apps no later than tomorrow.
Of course, total apps doesn't always tell the whole story as any Android user can attest, but the growth shows dedication from the developer community. The aim is not to hit a magical round number of apps in the store, but to hit a sufficient number of apps so there are no gaping holes in what users need. And, the Marketplace seems to be pushing towards that point.