Trying to sum count from multiple worksheets

Hi, I have a forecasting spreadsheet that has single worksheets per user to enter their requirements. I need to create a summary page which searches the content of each of the individual worksheets to locate if a part code has been selected on that page, and if so to include the count allocated to a specific month into the overall sum of that product for the month so we know what volume to order to fulfill requirements.

I'm not sure how to achieve this. I have uploaded a sample file to show the raw content, but I don't really know where to start in achieving the above solution. Thank you in advance for any suggestions offered.

Jo