It looks like you have a single row for customer, and a column for every month. This means, that you change the design of your table every month! And when you want to create some reports based on this table, then you have to edit them also in every month!


Instead consider more database-like design.
You will have a table with columns like Customer, Month, Value (Month being a date really - e.g. 1st of month - formatted like "yyyymm"), where you enter your data.
Then you can design report sheets, which read info from this table, and display it in any form you'll need.
And/or you can set autofilter on for data entry table, and add some SUBTOTAL formulas above your data entry table to calculate sum's or count's from filtered rows in data entry table.