The numbers may not be so important for most people though. Nielsen looked at the apps available for Android and found that the top 50 apps consume 61% of the time users spend in third party programs.
The blog post shows that just the top 10 apps capture 43% of "application time." I assure you, right now, Angry Birds is getting more than their fair share. That means applications 11-50 get 17% of application time and everything else, literally hundreds of thousands of apps, are fighting for that remaining 39%. It just shows you that having a popular app is a bit like winning the lottery.
Sure, there are a lot of apps in that "other" section that are handy, but how critical are they? It is nice to have a currency converter, but can't you get that on the Web? It might have been fun to play with that MyFord app to program an electric Ford Focus car, but that too is available on the Web. And honestly, how often to you use either of those or thousands--tens of thousands--of similar apps?
Windows Phone has a decent selection of apps, and there is no reason to think usage patterns differ. That doesn't mean Windows Phone will do everything an iPhone will. Banking apps are still a big missing component and an app I consider critical. It doesn't matter that you only spend 10 minutes a month with your banking app, that 10 minutes in invaluable when depositing a check via your camera and moving money out of savings into checking before swiping that debit card at the movie theater.
A key point to be taken from this data is app counts themselves aren't very meaningful, but app content is. If you have, say, three specific needs that are somewhat unique to you, like access to a specific bank, certain trail maps that you need, and a particular game, then you should check out the availability of those three apps and get a phone that supports them. It doesn't matter how many apps are available. Other than those three apps in my example, you'll probably spend over 60% of your application time in a tiny percentage of apps, like everyone else.