Hi,
I assume you on "sheet1" (total sheet) use the formula (example) =Sum(Sheet2:Sheet8!A1) to sum cell A1 through sheets2-8.

If you add new sheets between sheet2 and sheet8, they will automatically be included in the summary. If you however insert new sheets before sheet2 or after sheet8 it will not work anymore. You might also run into trouble if you delete sheet2 or sheet8.

Onw way to increase security is to create a sheet namned "First" to the left of sheet2 and one sheet called "Last" to the right of sheet8. The sheets "First" and "Last" should be empty, and all the sheets that should be summarized, should always be placed inbetween those sheets!

The formula should then of course be =Sum(First:Last!A1).

Kind regards
Bjorn