Try the following example:
1. Assuming you have a multiple row of Sheets where the first sheet is called "Firstsheet" and the last sheet is called "Lastsheet", plus you have a master sheet that follows on after the "Lastsheet"
2. You require to sum the cell 'A2' in all sheets between (and including) "Firstsheet" to "Lastsheet"
3. In the master sheet use the formula '=SUM(Firstsheet:Lastsheet!A2)' which will sum total all 'A2' cells in all sheets between and including Firstsheet:Lastsheet.
4. If at a future point you then add a new sheet between Firstsheet and Lastsheet, then its 'A2' cell will also be automatically included in the master sheet sum formula.
You can of course change / substitute your own sheet names in place of "Firstsheet" and "Lastsheet" names to match as appropriate. Just make sure that the master is sheet outside the sum sequence.