At least 22.
touch icon on home screen
touch widget on home screen
using a gesture
touch on all apps screen
touch item on previous used apps screen
double tap on previous used icon
touch item on notification screen
touch item that is a link on a web page
from within an app
from the settings menu
from a menu
initiated on another network connected device
automatically at a predetermined date and time
automatically by the phones manufacturers (updates)
Typing the apps name in a Search prompt
By touching on a file in a explorer screen
from a help screen
from a batch file
from a terminal screen
via an event happening. eg's data changes, another program stops, a temperature is exceeded
automatically when the phone starts
by using your voice and saying what you want to do