Answer:
Around 9, when we consider the most popular ones.
And those 9's are:
1. Android OS
2. iOS
3. BlackBerry OS
4. Bada
5. Palm OS
6. Symbian OS
7. MeeGo OS
8. Windows Mobile
9. webOS
Out of these 9, the most popular ones are certainly the Android and iOS. And you will find majority of mobile applications made either in Android or in iOS.
Step-by-step explanation:
The Android and iOS are the most popular operating system for mobile environment. However, others mentioned above are being used considerably as well, though much less than Android and iOS.