WHO KNOWS THE SOLUTION?

I have a choise field in a repair database: sort of customer: 1 or 2.
I book repairs for both customers 1 and 2.
All records have a unique repair number, specified by the customer.
From the booking date I select the monthly number via a formula field.
To create a dashboard, I want a breakdown of customers A and B. But also per month.
I have tried:
select repairs;
let i := (select repairs where Month = 11 and (select repairs where sort of customer = 1));
cnt(i.repair number)

Unfortunately, it doesn't work. Can someone help me on my way?

Linda