Hey!
I’ve been trying to storage a value and increment it each time a new form is completed.
But I don’t have any idea how to make a script that storage the amount or basically, take the amount given by the form, and sum it with the amount existing yet.
If someone can help me, please. 
to make the script is easy, but the question is where do you want to store the value? Do you have a doctype for this?
@johnskywalker yes I have a doctype. But it is not the same doctype that runs the data that I want to storage in it. In others words, I want to storage the data created in a doctype, to another doctype which will no be changed anywhere
is this a custom app and a custom doctype? If so you can do the script on its py file. If not you can do doc_events https://frappe.github.io/frappe/user/en/guides/basics/hooks