I'd guess somewhere between 2,000 and 5,000. People are going hell for leather right now expanding their portfolios. I'm now leaving names in my ToDo list rather than pushing them to design queue. Will wait for some more sales to justify more listings.
The bad news I've discovered is that sales rates are exceptionally lumpy across portfolios. I know of multiple users with 150+ names listed and zero sales to date.
Again, I suspect the site architecture and search feature may be killing sales of pure brandables, whereas keyword domains might be relatively easier to find via the site. Just a suspicion, but many things are pointing to this.
There#s very little other than guesswork to back this up, and I hope BB might share some aggregate stats to help us understand what's going on.
I've also sent some ideas for improving the Name My Company app which, based on a cursory review today, is doing exactly what I thought it would - lots of submissions, very little voting, and no incentive for BB sellers to find the best name for any competition. I hope they can improve this as it could be a great feature for both buyers and sellers if well implemented.