@Nova31
An alternative to the Power Query solution that @Sergei Baklan suggested--not all of us have access to Power Query (I don't on my Macs)--is to rethink your workbook.
I worked not too long ago with an acquaintance whose small business has several hundred SKUs, and, like you, some six or seven major customers. We integrated ALL of his order records into a single table--i.e., every customer's orders mixed in with the others--by just adding a column that identified the customer ID or name.
I'm assuming your six distinct sheets are laid out in very much the same way, perhaps identically. It should be quite easy to copy and paste into a single consolidated database (ie., a table), ensuring first that you've included a column to identify each row of each transaction with the appropriate customer
Once you've done that, you can use the Pivot Table or possibly FILTER function, or SUMIF to extract very tightly defined summary reports by customer, by year, by category of SKU...whatever. It's simply a LOT easier to produce summary reports from a single table or database.
As Sergei noted, if you want more specific help, it would help if you could supply a sample of your spreadsheets....just don't include proprietary data.
