At least 22.

Here is, most likely an incomplete list of them:

1. touch icon on home screen
2. touch widget on home screen
3. using a gesture
4. touch on all apps screen
5. touch item on previous used apps screen
6. double tap on previous used icon
7. touch item on notification screen
8. touch item that is a link on a web page
9. from within an app
10. from the settings menu
11. from a menu
12. initiated on another network connected device
13. automatically at a predetermined date and time
14. automatically by the phones manufacturers (updates)
15. Typing the apps name in a Search prompt
16. By touching on a file in a explorer screen
17. from a help screen
18. from a batch file
19. from a terminal screen
20. via an event happening. eg's data changes, another program stops, a temperature is exceeded
21. automatically when the phone starts
22. by using your voice and saying what you want to do