We've been using the flexDrive (wiki) for over a year now in the Moore lab, recording almost 100 sessions in 5 mice. I'm just now starting to analyze neural ensemble statistics that require simultaneously recorded neurons.
Here's the real-world distribution of how many simultaneous neurons in primary somatosensory cortex (with some thalamic electrodes) I could sort over a total of 75 sessions in awake mice with 16 nichrome tetrodes.
The mean unit yield was 25.8, with a minimum of 8 and a maximum of 46 units. These numbers include some not so great recordings, and bad tetrodes hat got damaged etc., but only very few sessions were outright discarded, mostly in the beginning of the drive lowering process where it looked like some electrodes were not in cortex yet.
All in all, these numbers should be good enough to do some interesting assembly-analysis, though the relatively low density of the tetrode array (250 micron pitch) results in a relatively low occurrence of strong fast-timescale correlations between spike trains.