With the Increment Value action you can increment or decrement a number by any value that you want.
All you need to configure is which column the value you want to increment is in and then how much you want to increment by. If you want to decrement a number then simply add a minus before (e.g. -1).
You can choose a custom value to increment by or you can choose a column in your sheet. When you choose a column in your sheet for the increment value, this means the number you increment or decrement by can be dynamic.