Not actual zombie games but 'zombie' games. I.E. a game that doesn't even rank on Store listings. As of June the number's hit a new peak.
According to this report (via Slashgear) of the 1,197,087 apps on the Store in June, 953,387 were something "that does not attract enough measurable attention to regularly receive rankings in the Apple App Store top list". Basically 79.6% of the stuff on the Store right now is just rotting away in obscurity.
To put it another way, four fifths of apps on the Store simply aren't seen, downloaded or even acknowledged. That's a lot of shit. It's due in part to the shear volume of games. In 2013 453,902 new apps were released and it's predicted that 578, 000 more will hit by July 2015.