There is an obvious App gap based on numbers only for clear as day reasons, but with the availability of alternatives that gap is closing somewhat in WP. The main talking point shouldn't be about the number of apps between OSes but the quality of Apps between them. I couldn't careless if both Android and IOS had 500,000+ apps more than WP, what I and I hope you too as well want is that the QUALITY/FUNCTIONALITY from the apps found on all 3 OSes are the same in look and operational usage. I've recently downloaded flightradar24 on my WP8 L920 because I work at the Airport in my Country in the Caribbean and I found that it's close but not touching distance close to the one found on a work-mate's IOS version. As he turned his on every plane that was in the air showed up almost immediately on his screen complete with tail tags, on mine it took a while for about less than 10 to show up in the same area and I don't have the tags to show just by looking at the screen who was who. I bought add ons but his was more functional as like I said he had tags to say which was what. That's an example of how the quality difference is between the apps found on android and ios and those found on WP. Another example i have at work is facebook messenger. The one found on android (Sony Xperia Z1) has a phone icon whereas none found on the the WP version of the latest updated facebook messenger, so there's another example as to it's MORE important to have the QUALITY of app experience more than having the QUANTITY of app experience as most of those same 000,000+ apps are hardly ever used!!!!
Just my 2 cents.