Personally, if it's just incrementing the variable each time, I'd do:
Keyboard: on key pressed 1
-System: Add to Variable 1
If you need to limit the variable, you can use the clamp expression. For example, if the highest the value could be is 2:
Keyboard: on key pressed 1
-System: Set Variable to Clamp(Variable+1,0,2)
If you need to roll the variable back to 0 when it goes past 2:
Keyboard: on key pressed 1
-System: Add to Variable 1
System: Compare Variable > 2
-System: Set Variable to 0
etc...