This is a great “Summary Page” technique. All of your sheets must be laid out in a consistent layout. The following SUM formula will summarise the data from all sheets onto a summary sheet.
=SUM(Sheet1:Sheet3!C5). This will add the contents of cell C5 on all sheets in the range, in this case Sheet1 to Sheet3.

This is a powerful technique and here is a tip to make it easier.
Set up your sheets in the normal way..

1) Insert a new sheet to the left of your sheets and rename it START;
2) Insert another new sheet at the right of your sheets and rename it END;
3) Use this formula: =SUM(Start:End!C5)

You should leave the START and END sheets blank. I would advise that you do not hide these sheets if you are sharing the Excel file with others

You can now add or remove sheets from this range as you need to.

Whenever you insert a new sheet tab into this workbook it will be added between the START and END tabs and will be automatically included in your summary sheet formulas.