It seems that on the average we are using no more than 20 apps. I've got over two hundred on my iPhone/iPad but, indeed, the ones that I am using regularly are around 50 and the ones I am using every single day are less than 20.
Yet, developers still develop new apps. According to TechCrunch there will be close to 3 million apps in the Apple Store by the end of this year and there will be 5 million apps by 2020. These forecast is based on projections from SensorTower.
Most importantly, these numbers refer to the apps in the Apple Store, not to the ones that have been developed. Their number is significantly greater since many apps do not succeed and fade away.
Almost half of the apps are games related (in May out of 48,000+ apps published in the store around 21,000 were games), business apps, education apps, entertainment apps, lifestyle apps are each below 5%.
With this huge and growing number of apps the problem of finding an app is getting bigger and bigger. We should expect some smart search engine to become available in the future. I would also expect to see Google answering one of my search by pointing me to the existence of an app that can be useful to be here and now. Not sure how Google, Apple, MS are really going to work together to make this feasible from a biz point of view.
Also, and we already see this happening, the more apps users have the less time is available to use each of them. This is clearly a challenge for apps developers. Again, I would expect some smartness in future operating systems that will automatically pop up an app as the situation demands.